Basic Rules for understanding Vaisnava Calendar.
1. The Tithi at sunrise rules the day. 2. If the Tithi begins after sunrise
and ends before sunrise of the next day (Lost) it is combined with the next Tithi. 3. 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.
Rules for Ekadasi :
Srila A. C.Bhaktivedanta
Swami Prabhupada makes an interesting comment in his purport to Sri Caitanya Caritamrta Madhya lila. 9-24-342: "You should
recommend the avoidance of mixed Ekadasi and the performance of pure Ekadasi. You should also describe the fault in not observing
this. One should be very careful as far as these items are concerned. If one is not careful, one will be negligent in executing
General Rules for observance are:-
1. Ekadasi must
come in prior to the Arunodaya Viddhi (1hour, 12 mins, 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 Dwadasi ( making Mahadwadasi
or Compounded) on the next day. 2. If Ekadasi falls on Sunrise two days in a row, fasting is observed on the second day.
(If Mahadvadasi is in force, fasting is prohibited on the Ekadasi,
but the observance is done on the Dwadasi. Compound Dwadasi-Mahadwadasi.) 1. 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
Mahadwadasi. 2. If Dwadasi begins after sunrise and ends before sunrise on the next day (Trayodasi), it is also Lost, and
is to be observed as Tri-Sprsa-Mahadwadasi. 3. If Dwadasi falls on the Sunrise two days in a row the first Dwadasi becomes
Vyanjuli Mahadwadasi. 4. When the following Amavasya or Purnima falls on sunrise two days in a row the preceeding Dwadasi
becomes Paksa-Vardhini-Mahadwadasi, Paksa based. Parama Ekadasi is in the month of Purushottam.