Please see the letter below for information on this particular calendar event
Basic Rules for Determining Ekadasi and Mahadvadasi
Srila Prabhupada Letter to Madhudvisa prabhu 69-09-30 England:
“Regarding you first question, we observe Ekadasi from sunrise to sunrise. The 12 midnight is western astronomical calculation, but the Vedic astronomical calculation begins either from the sunrise or the moonrise. Generally it is sunrise. Our calculation is like this: when the sunrise is there, Ekadasi tithi (date) must be there. If Ekadasi tithi is not in the sunrise and the tithi begins, say after a few minutes after the sunrise, then we accept that day as previous to Ekadasi. All our ceremonies are calculated in that way. This means we must see the tithi during sunrise. Therefore, sometimes our dates of ceremonies do not exactly coincide like the western calculations. Just like Christmas Day they have fixed up on the 25th December, but our Janmastami tithi is not fixed up like that. My birthday is on the 1st September, 1896, but this year the tithi of my birthday was fixed for the 4th September. So it is very difficult to calculate, therefore we have to take help from the Indian expert almanac astronomers.”
The tithi at sunrise rules the day
– If the tithi begins after sunrise and ends before sunrise of the next day (lost) it is combined with the next tithi.
– If the same tithi falls on sunrise two days in a row, observances are scheduled on the first day; except Ekadasi, Amavasya, or Purnima tithis which are scheduled for the second day.
– Ekadasi must come in prior to the brahma muhurta (1 hour 36 min before sunrise). When it does so it is called Suddha Ekadasi, pure Ekadasi. If it begins after that, it is considered impure and is therefore to be observed on the Dvadasi (making Mahadvadasi or compounded) on the next day.
– If Ekadasi falls on sunrise two days in a row, fasting is observed on the second day.
– If Ekadasi begins after sunrise and ends before sunrise the next day it is considered Lost, too short, thus not full, and impure. Therefore the next day is called Unmillani Mahadvadasi.
– If Dvadasi begins after sunrise and ends before sunrise on the next day (Trayodasi), it is also lost, and is to be observed as Trisprsa Mahadvadasi.
– If Dvadasi falls on the sunrise two days in a row the first Dvadasi becomes Vyanjuli Mahadvadasi.
– When the following Amavasya or Purnima falls on sunrise two days in a row the preceding Dvadasi becomes Paksa-Vardhini-Mahadvadasi, Paksa based. Parama Ekadasi in the month of Purusottama.
– For more details see Navadvip Panjika intro of Srila Bhaktisiddhanta Sarasvati Thakura.