A brAhman is called a ‘dvijan’ ie one with two births. Just like how UpAkarmA is considered very important for a brAhman, ChaturmAsya Sankalpam is considered very important for a SanyAsi (yathivara). The number of chaturmAsya sankalpams observed by a yathivara is used a measure of a sanyAsi’s age in thuri-Ashramam (i.e. after becoming a sanyAsi) and not his actual age. Our shAstras require that if a yathivara who is of older age meets a younger yathivara who has observed more chaturmAsya vrathams, the older yathivara should prostrate in front of the younger yathivara. This is because the number of chAturmAsya vrathams observed by the younger yathivara is more than the older yathivara and hence is considered elder in position.

In the 4 months of Aavani, Purattasi, Aippasi and Karthigai, it normally rains heavily in India. This helps in enormous procreation of many insects and other tiny living creatures in the soil during this rainy season. Because they are everywhere on the ground, they are harmed immensely by the people treading upon them and even gets killed. As Thiruvalluvan beautifully summarizes the yathi dharmam in a KuraL: "uRRa nOy nORRal Uyir KurugaN Seyyamai, aRRe thavathiR guru" about a sanyasi's dharma, the sanyAsis have undertaken an “ahimsa dharmam” towards all living beings, they avoid any harm to these insects and worms by stopping all sanchAram in these months and by taking a vow to stay in that place for this 4 month period. This vow is called ChAturmAsya Sankalpam.

There is a veda vAkyam which says “pakshA vai mAsA:” i.e. a paksha (15 days) can be considered as a month. Since it is very difficult nowadays for yathis to stay in a place for 4 months due to their sishyas wishes for their achArya to undertake sanchAram constantly and reach out to them, the sankalpam is nowadays undertaken by yathivaras for 4 pakshams i.e. 2 calendar months instead of 4 calendar months. This vratham is generally started on the Pournami following Aani- AmAvAsyai and completed on the Pournami day following Aavani-AmAvAsyai.

On the day of the beginning of this vratham, the yathivaras complete their nithya karmas and bhagavadh arAdhanam. A small piece of soily land will be consecrated and purified with AvAhanam and aradhanam to Sri Boovaraha Perumal and Sri Bhoomi Piratti. Sishyas will congregate in this place and chant Bhoo sooktha veda parayanam during the aradhanam. After the ArAdhanam, the achArya yathivara will collect this soil himself  required for their anushtAnams during the period of the vratham. After this, the yathivara will undergo thirumudi viLakkam (shave), will have bath and then perform special aradhanam again. After the ArAdhanam, yathivara will be joined by all the sishyas in reciting pancha shanthi. After the completion of this veda parayanam, both the AchArya and his sishyas will undertake a sankalpam (vow). First the AchArya will say

“AashAda PournamAseem Arabhya bhAdrapada PournamAsi paryantham ChaturmAsya vratham sthAsye”

meaning “I undertake this chaturmAsya vratham from Aadi Pournami till Aavani Pournami”

After the Acharya undertakes the chAturmAsya sankalpam with the above words, the sishyas gathered will tell the AcharyA the following verse

“Nivasanthu sukhenAthra gamishyAmaha kruthArthathAm !
 YathAsakthi cha sisrooshAm karishyAmO vayam mudha !!”

meaning “You (the AchArya) please kindly stay in this place with all happiness and bless us for an opportunity to serve you. We vow to serve you to the best of our abilities”

The significance of starting the sankalpam on Aashada Pournami is that it is celebrated as the avathAram day of the great sage VyAsa who organized the Apourusheya Vedas into 4 major sections.(Rig: 1131 Sakhas or Recensions divided into Rig (21 sakhas), Yajur: 101 sakhas, Sama: 1000 sakhas and Atharvana Veda: 9 sakhas); wrote the Brahma Sutras (555 Sutras); wrote 18 Maha Puranas which includes Srimath BhAgavatham (Brahma Purana, Padma Purana, Bhagavata Purana, Siva Purana, Skanda Purana, Garuda Purana, Brahmanda Purana etc. Of these 18, Vishnu Purana was wrote by Vysacharya's father Parasaracharya, but was edited and presented by Vyasa). He is also celebrated as “VyAsAya vishnu rupAya vyAsa rupAya vishnave” in Vishnu Sahasranam and by our Swami Desikan as “mey alladhu viLambhadha vyAsan kAttum vilakillA nan neRiyE viRaindhu selveer”. Such is the great significance of this chAturmAsya sankalpam day.

Shaka vruta will end with samarpana mantra:

           Upaayanamidham Dheva Vrutha sampoorthiHethaVe
           Shakam thu Dhvijavaryaaya sahiranyam dhadhamyaham.

As you have mentioned in dhadhi vruta curds(yoghurt ) is not allowed for consumption but buttermilk is permitted. Here there is an important aspect which should be noticed is even though we can use buttermilk the butter from the curds should be removed by churning it thoroughly before using it.  Then it will fulfil the purpose of dhadhi vruta. Dhadhi vruta sankalpa mantra:

            Dhadhi Bhadhrapada Maasi varjayishyE sadha harE
            Imam karishyE niyamam nirvignam kuru keshava.

In Kshira vruta milk and food prepared using milk like khova etc... are not permitted.  Kshiravruta sankalpa mantra:

            Kshiravrutamidham DhEva GruhItham purathasthava
            Nirvignam sidhdhiMaayathu prasaadhaththe ramaapathE.

More information from the Madhwa line citing all manner of PurANas HERE:

Restricted foods during the entire four months of Caturmasya:

Chaturmasya: Vows and Benefits

Gaudiya Math:
Iskcon's: Standard: by Nayana-ranjana Dasa
Brahma Madhwa Vaishnava followers of Madhwacharya:

Iskcon's Standard:
"One who passes the Caturmasya season without observing religious vows, austerities and chanting of japa, such a fool although living should be considered to be a dead man." (Bhavisya Purana)

If one can't follow the things given below, at least one should follow the following basic rules:
 

"In the month of Sravana (July, August) one should refrain from eating spinach. In the month of Bhadra (August, September) of yogurt, in Asvina (September, October) milk. In Karttika (October, November) meat and urad dahl." (Skanda Purana, Nagava Khanda)

Various vows and austerities during Caturmasya and their respective results:

"O King, one who is My devotee and is fixed in vow, whether man or woman, for the purpose of dharma, should accept these various austerities and observances. I shall now describe to you all of them along with their respective results.

01) No salt - One's voice becomes sweet.
02) No oil - One's life is prolonged and gets progeny.
03) No oil massages - One's body becomes beautiful.
04) No cooking with oil - One's enemies are vanished.
05) No licorice and oil - One becomes wealthy.
06) Give up wearing unoffered flowers - One becomes a Vidyadhara in Devapura.
07) Give up six kinds of tastes (spice, sour, bitter, sweet, salty and harsh) - One never becomes ugly, smelly, or obtains a bad birth.
08) Practice of yoga - One goes to Brahmaloka.
09) No betel nuts - One becomes happy.
10) No cooked food (raw fruits and vegetables) - One obtains purity.
11) No honey - One becomes lustrous.
12) No yoghurt or milk - One attains Goloka.
13) No cooking with earthen pots - (?)
14) No hot food - One gets offspring with a long life.
15) Take rest on the ground or on stone - One becomes an associate of Visnu.
16) One who gives up meat and honey is a yogi and muni.
17) No intoxicating liquors - One becomes powerful and free from disease.
18) Fast for one day - One is honored in Brahmaloka.
19) No cutting of hair and nails - One gets the benefit of taking bath daily in Ganges.
20) No speaking of grama katha (nonsense) - One's order will never be disobeyed.
21) Eat food on the ground without dish or plate - One obtains a kingdom on Earth.
22) Chant the mantra Namo Narayana - One obtains the result of giving in charity 100 times.
23) Offer prayers to the Lord - One gets the result of giving cows in charity.
24) Touch the lotus feet of the Deity - One becomes a successful person.
25) Clean the temple of the Lord - If one is king he remains so for a kalpa.
26) Circumambulating the temple three times offering prayers - At the time of death one will mount a swan airplane and go to Vaikuntha.
27) Singing or playing musical instruments in the temple of the Lord - One goes to Gandharvaloka.
28) Take pleasure in studying the sastras - One goes to Visnuloka.
29) Sprinkle water in the temple - One goes to Apsaraloka.
30) Take bath in a holy place - One's body becomes pure.
31) Worship Lord Visnu with flowers - One goes to Vaikuntha.
32) Eat pancagavya - One gets the result of observing candrayana fasting.
33) Eat one meal a day - One gets the results of performing an Agnihotra.
34) Eat at night only - One gets the result of going to all the places of pilgrimage.
35) Eat at noon only - One attains Devaloka.
36) Take food not obtained by begging or any extraneous endeavor - One gets the results of opening water stands and digging wells.
37) Take bath daily - One will never see hell.
38) No eating on a plate - One gets the result of bathing at Puskara.
39) Eat on a leaf - One obtains the result of living at Kuruksetra.
40) Eat on a stone - One obtains the result of bathing at Prayaga.
41) Giving up drinking to six hours - One will not be attacked by diseases.
42) During the Caturmasya, garlands, caranamrta, candana and water from the conchshell of the Lord, mahaprasadam, Deity garlands must be accepted.

"In this way one who performs this vrata for the satisfaction of Lord Kesava becomes satisfied himself. O best of the Pandavas, among My devotees, he who accepts these vows and austerities during the Caturmasya goes to My abode at the time of death, of this there is no doubt."

Visnu Rahasya (Lord Brahma to Narada Muni): "If one accepts all these Vaisnava austerities and observances with devotion, one attains the supreme destination, O Narada. One who desires within himself to execute all these Vaisnava austerities and vows, his sins obtained within one hundred births are destroyed. If one is exclusively devoted, peaceful, takes daily bath, is fixed in vow and worships the Lord during these four months, he goes to the abode of the Lord. He who accepts the austerity of sleeping on the ground this time when the Visnu Deity is sleeping in yoga nidra underwater, he goes to the abode of Laksmi."

Gaudiya Math:
http://gaudiya.net/purebhakti/calendar/caturmasya.htm

Restricted foods during the entire four months of Caturmasya:

- Tomatoes, eggplants, runner beans, loki, parmal, urad dahl and honey.

Particular foods that are restricted during each of the four months:

First month: No leafy vegetables such as spinach, salads of all types, cabbages of all types, kale, leafy herbs such as coriander, mint, parsley, curry, and powdered leafy herbs.

Second month: No yogurt (if one requires for health, it can  be mixed with water)

Third month: No milk (if required, it can be mixed with lemon juice).

Fourth month: No mustard oil nor sesame seeds.

Foods that can be taken all days of the year, including Ekadasi and Caturmasya:

-All fruits (fresh and dried), all nuts and oils made from nuts.

-Potatoes, pumpkin, cucumber, radish, squash (but no loki), green papaya, lemon, jackfruit, avocado, olives, coconut, buckwheat, all sugars.

-All milk products except pure milk and yogurt during Caturmasya and sweets with starches on Ekadasi.

The Greatness of ChAturmasya

By upanyAsa RatnAkara, Sastra viSarada Shri Rama Rao B.A., L.T, Kumbakonam, TN

Nov ’77 issue of Dharmaprakash Journal of the Madhwacharya Vaishnavas. With kind permission by Sri K Raghupathy Rao

The ChaturmAsya is important not only to the ascetic but to all. The word Yati, though it ordinarily means ascetic, etymologically means all those, who strive for salvation by performing the duties enjoined on them by the Sastras and dedicating the results thereof to God.

Atharvana 6.6: sanyAsa yogAt yatyaS SuddhasatvAH

The purAnAs and dharmasAstras describe in detail the greatness and significance of the quadruplet of months. Among the purAnAs, the VarAha, the skAnda and Aditya are chief.

Asked by his consort, the goddess of earth, Sri VarAha replies as follows:
“The so-called four months are the most beloved period of mine. It is one of baths and vows, japas and homas, gifts and sacrifices. What is done in MAgha is a 1000 times greater than what is done in other months, and that done in VaisAkha is a thousand times greater than that done in MAgha and that done in CAturmAsya is definitely greater than that done in VaisAkha”.

The six months of UttarAyaNa are the daytime of gods and the months of DakSiNAyana are their night time. Once, the gods, who had assembled before the Lord Sri Hari on Mount Meru, took leave of Him as their night set in. Just then, the goddess of their night (DakSiNAyana) in the guise of a lady of dark complexion, with an axe in hand fell at the feet of god and said, “Oh merciful one! UpanayanAs, marriages and other auspicious celebrations are banned during my period; I am branded as an inauspicious one and censured by all. I pray that I may be spared or I shall end my life”.  The gods too pleaded on her behalf and Lord Sri Hari was pleased to grant her the following boon:

“The first 4 months of hers; viz., SrAvaNa, BhAdrapada, Asvina and KArtika shall be more auspicious than all the other months. Any good deeds like japa, vow, gift and so on done in these months shall yield an infinitely greater merit than done in other months. Even in these four, each day is more meritorious than the previous and shall reach its climax in KArtika”.

The gods and goddess of DakSiNAyana were all pleased and went to their respective places. From then, the CaturmAsya is hailed supreme and so we must make the best use of this period and be benefited in all ways.

The CaturmAsya commences on the Sukla ekAdasi of ASADha and concludes on the Sukla ekAdasi of KArtika. Lord Sri Vishnu (upendra) is supposed to be sleeping throughout, with His consort Lakshmi on the bed of Sesa in SvetadvIpa, of the ocean of milk. As he goes to bed on the Sukla ekAdasi of ASADha, it is called sayana ekAdasi; As he turns on His side on the Sukla ekAdasi of BhAdrapada, it is called Parivartana ekAdasi and when He wakes up on Sukla ekAdasi of KArtika, it is called UttAna ekAdasi. [A note of caution has to be sounded about His sleep. It is not like our sleep, which is the outcome of tamoguna].

Aditya purAna – Brhama nArada samvAda.
Lord Sri Vishnu has given up cereals in the first month, curds in the second month, milk in the third and dicotyledons in the fourth, since all the sins rest in them in those periods. Hence we should not eat these things given up by Him.

On the Sukla ekAdasi of ASADha, Sudarshana homa must be performed and we should have mudrAdhAraNa. We should take the four-fold vow before God: “Oh Lord of the worlds! When you go to sleep, the world will sleep and awake when you awake. Be pleased with me. During the 4 rainy months and till your waking up, I shall give up cereals in SrAvaNa, curds in BhAdrapada, milk in Asvina, and dicotyledons in KArtika. I take this vow before you and let it be fulfilled without any obstacle by your grace”.

We may take up other vows also, such as, giving up jaggery, oil etc., making an honest confession of our sins and in a penitent mood we may take the vows of Gopadma Vrta, lakshapradaksina, LakshanamaskAra and so on. Having observed all the vows, we must dedicate them to God on the Sukla dvAdaSI of KArtika (utthAna dvAdaSI), praying for His pardon the lapses and ensure the perfection of the same.

Violation and non-observance of the above 4 vows is condemned in strong terms as follows: One who eats cereals in the first month strikes an arrow into the heart of Vishnu; taking curds in the second month is taking beef; drinking milk in the third is equivalent to drinking wine; eating dicotyledons is equal to eating worms in the fourth.

However, the sAstras are considerate and have made provisions for the expiation of the sins caused by our non-observance of the vows in the form of vanabhojana in the KArtika month.

SanyAsin and ChAturmasya:
As they have given up all attachments, the sanyAsin should not stay in one place for more than a day. In their daily movements from place to place, they may find no time for the japa, pooja, sAstraic studies, upadesa, meditation and so on. To facilitate these, they are permitted a four month stay in one and the same place. Secondly, the ChAturmasya period is rainy season, which makes touring hazardous, and various worms and insects may be trampled under our feet. This is against the harmlessness creed of the sanyAsin’s in thought, word and deed. They must repair to a selected place and have the preliminary ceremonials of mrttika bath and perform pooja and declare before the gathered elite, ‘we shall live here for the benefit of all the creatures. Generally, during the rainy season the path is infested with worms and insects. Without any harm to them we shall stay here for 4 months enjoined by the sAstras’. The elite of the place prostrate before him and say in reply, ‘you may do so, Oh sire! I and we are all blessed. We shall gladly be ever at your service and worship you by all means’. The observance of ChAturmasya is a must for every peetAdipathi. Sages have observed it from time immemorial, with all ritual. The significance of its observance is mainly to acquire mightier spiritual powers, for the benefit of mankind.

Thus ends the greatness of ChAturmasya.

AcAryAh SrimadAcAryAH
Santu me janma janmani

[As always, erors if any, are mine. Learned members please pardon and correct]

Keshav Bhat

http://www.dvaita.org/list/list_02/msg00014.html

Dear Haribhakthas,

Today, first Ekadashi of Aashada is also the first day of Chathurmasya. For
the next four months (until the first Ekadashi of Karthika) it is believed
that Lord goes into a "yoga nidra" (Spiritual sleep?). During the next four
months Maadwas practice some strict vrathas:

a) Follow strict diets which don't include fruits/vegetables during first
month, curd during second month, milk during third month and
beans/lentils/seeds, etc during the fourth month.

b)  Perform some vraths like Laksha (100,000) Deepa, Laksha Namaskara,
Gopadma, etc.

c) On this very first day, the swamijis of respective mutts perform
mudradharana to all devotees. Also, they will not travel.

I would appreciate if some one in this group can help me understand:

a) What is exactly meant by Lord's Yoga Nidra?

b) Why is it these four months so auspicious for Maadwas?

c) What is the reason behind these diets?

d) Are there any particular Purana which discusses about Chathurmasya?

Thanks in advance for your information,

Hari Sarvothama Vayu Jivothama!

Murthy

Chaaturmaasya (Madhwas' vrata)

On the full moon day of aashaaDha, the paryaaya Swamiji takes a ceremonial shave and this is the commencement of chaaturmaasya or staying in a fixed place for a period of four fortnights.

After the daily rituals are over, the Swamiji holds a tray containing sacred clay and firewood, and addresses the assembled devotees thus.

      praayeNa praavR^ishi praaNi saN^kulam vartma dR^ishyate  |
      atasteshaamahimsaarthaM pakshaavai shrutichodanaat.h   |
      sthaasyaamashchaturomaasaanatraivaasati baadhake  ||

       It is the rainy season. The paths are full of insects. We shall
       restrict our movements so that they are not hurt. As per a
       statement of Shruti, a fortnight (of scriptural practice) is
       considered as equal to a month. We propose to spend four
       fortnights here in a fixed place so that no harm be caused to
       living beings.

To this the householders present reply, `Please do stay here and grace us by your presence.'

During the next two months the scholars chant the Vedas, Ramayana, Mahabharata, Bhaagavata and the works of Sriman Madhvacharya.

Now-a-days, instead of the full-moon day of aashaaDha, the chaaturmaasya starts from the fifth day of the dark fortnight of aashaaDha. This coincides with the anniversary of passing of Sri Tiikaachaarya (Sri Jayatiirtha).

During the Ekaadashii fasting days of this period a special worship known as jaagara puujaa takes place in the nights. After the raatri puujaa the Swamiji carries on his head a tray containing a tuLasii. He dances chanting the glory of the Lord. The musicians and attendants dance and sing devotional songs and then the scholars chant the sacred epics until late into the night. (Ekadashii nights are meant to be spent awake in the study of scripture and chanting of prayers, etc., in addition to the whole day and night being spent fasting.)

Special discourses on the sacred epics are arranged in three places:
(1) the platform in the Madhva-sarovara,
(2) the room known as simhaasana in front of the seat known as the Madhva-piiTha and
(3) in the candrashaalaa near the southern door of the shrine of Hanuman.

These arrangements are made so that the chanting of the sacred texts can be heard from the time the devotees take their bath in the Madhva-sarovara until they leave the temple after taking the darshana of Sri Krishna.

Apart from the chaaturmaasya, the chanting of sacred texts takes place every day in the chandrashaalaa. As well as this, special chanting takes place in the simhaasana. They also take place everyday throughout the year in the chauki during the time of the mid-day meal.

Shri kRuShNARpanamasthu

More information from the Madhwa line citing all manner of PurANas HERE: