So we can understand from this there there are two separate instructions (i) for ekadasi (ii) for Gaura Purnima and Sri Krishna Janmastami.
There are several quotes particularly from the earlier years regarding observance of ekadasi fasting on days such as Janmastami and Gaurapurnima.
The first thing to understand is that Gaura Purnima and Janmastami are
totally different things from Ekadasi.
On an Ekadasi if one takes grain then all manner of sin comes down upon
him, as sin personified - the Papa Purush resides in grain on the Ekadasi
tithi. (More about Ekadasii from the Hari
Bhakti Vilas Vaishnava Smriti)
On such observance days as Gaura Purnima and Janmastami among the Vaishnavas
it is the proceedure to fast entirely, or to take just water on these sacred
days, up until the evening when the Lord appeared. Normally the devotees
would finish their vrata (austerity), maybe take some water or some even
continue to fast until the next day. Then on the following day there
would be the utsav (festival - including feasting) to observe the advent
of the Lord (ie. Nandotsav after Janmastami, and the festival of Jagannath
Misra after Gaura Purnima). So feasting is done the next day, it is not
that one HAS to even take foodstuffs on the vrata day.
The term Ekadasi feast or Ekadasi prasadam would be more correctly termed
Ekadasi sytle prasadam, meaning without grain(s).
There is a discription of Gaura Purnima in Mayapur India 1976 the final Gaura Purnima when Prabhupada was still present from the biography "A Transcendental Diary" Vol1 Mar.16. by Hari Sauri Prabhu: pg.461. "GAURA PURNIMA. The Appearance day of Sri Caitanya Mahaprabhu. Everyone fasted until moon rise and then took an Ekadasi feast, although Prabhupada said this was not compulsory. (meaning one doesn't have to eat anything...... one may go one and fast as everyone else in 'India does!!!)
This is an interesting point, and reveals one's intentions upon the conclusion one draws. We weak minded, and sensually lusty personalities find it hard to fast, even a few hours. And so, Prabhupada being extremely merciful, made an exception and provided some small prasadams that didn't consider breaking a fast if taken - fruits or boiled potatoes...(see below)
Suggestions to the devotees:
"So far the Advent Day of Lord Caitanya is concerned, I have written
a full suggestion to Montreal, in which the main points are that
we should all observe strict fasting up till moonrise, and at that
time, an offering is made to Lord Caitanya of Ekadasi foods, fruits, peanuts,
milk, and so forth. Then, on the next day, Friday the 15th, a full-scale
feast is held to celebrate His Advent Day. On the 14th, chanting, reading
of Srila Bhaktivinode's book, Life and Precepts and Caitanya Caritamrta,
Introduction to Bhagavatam, may be held all the day in the Temple."
(San Francisco, 12th March 1968 letter to Balai)
"Lord Caitanya's Appearance day falls on March 22 (1970), that is on a Sunday. The devotees should fast until evening, when there is a ceremony and offering of a small amount of Ekadasi preparations. The next day, the devotees should celebrate amongst themselves with a small scale feast. You may hold the celebration open to the public on the following Sunday. The preparation to be offered specially on this occasion is bhuni kichri: Fry equal parts dal and rice in ghee and massala. Add two times water as dal and rice, and add vegetables (if you use more ghee, use less water). Cook it until it is dried and the rice is soft."(Letter to Krishna dasa. 18th February 1970. Los Angeles.)
"P.S. No feasting in the evening of Caitanya's birthday. Fasting all day till evening moon rise. Then take Prasadam like Ekadasi. Next day you can have full swing feasting. On the birthday if can take a procession of Kirtana on the street it is very nice." (Post Script to Letter to Rayarama dasa. September. Unknown place unknown date Ref:91-05-14.)
"You can take quotations from the above mentioned books, and have discussions. Fasting should be continued up to evening, then you may have food like on Ekadasi, fruits, milk, etc. On the next day you may observe festival. Janardana consulted me and said he thinks Sunday feasting would be best, so you may do that if you like. Or you may do both days, as you feel best."(Letter to Mahapurush. 7th March 1968. Regarding Gaura Purnima)
Proof that Srila Prabhupada followed this too:
"We performed Lord Caitanya's Birthday ceremony, not very gorgeously,
but amongst ourselves, and Govinda dasi cooked very nice Ekadasi foodstuffs.
The next day I expected some big ceremony but it couldn't be performed.
(A.C. Bhaktivedanta Swami Prabhupada. Letter to Purusottam. 6th March 1969.
Hawaii)
There is an amusing discription of the first ISKCON janmastami "feast" as described by Hayagriva Prabhu in his biography "Hare Krishna Explosion pg 62" He writes:"as midnight approaches, we hungrily envision the great birthday feast of Lord Krishna, mountains of succulent prasadam: cake and kachoris, halava and puris, sabji, sweet rice, samosas and gulabjamuns. Just a few minutes before midnight, Swamiji finally descends from his kitchen with the Prasadam. But our faces drop. There is only a platter of cut fruit."
There is a discription of Gaura Purnima in Mayapur India 1976 the final Gaura Purnima when Prabhupada was still present from the biography "A Transcendental Diary" Vol1 Mar.16. by Hari Sauri Prabhu: pg.461. "GAURA PURNIMA. The Appearance day of Sri Caitanya Mahaprabhu. Everyone fasted until moon rise and then took an Ekadasi feast, although Prabhupada said this was not compulsory.(meaning it was not compulsory to break the fast with anything but to continue fasting until the next day)
Pg.466: "Many Devotees sold copies of the Gitar-gana, and mass distribution of Halava prasadam went on throughout the day."
Pg.469 "At one point Prabhupada sent me out to see how many visitors had come , and he was very happy to hear of the large crowds. Typically, he wanted assurance that prasadam was being distributed to all."