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.
be very careful as far as these items are concerned.
If one is not careful, one will be negligent in
executing devotional service."
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.
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.