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 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.
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.

Mahadwadasi :

(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.