Mahadvadashi / Types / How to Determine

Sometimes, due to the manner in which the ekadashi tithi and the dvadashi tithis are positioned in relation to the sunrise the vrata is observed on the dvadashi and the fast is broken the next day, the trayodashi. Such a fast day is called a mahadvadashi, to distinguish it from a regular dvadashi, which is the day to break the ekadashi fast.

Unmilani-mahadvadasi

If the ekadasi tithi begins by the arunodaya of one day and continues until or past the sunrise of the next day, i.e. the ekadasi is vrddhi or increased, the second of the two days will be called unmilani-mahadvadasi. On that day one should observe the ekadasi vrata.

Vyanjuli-mahadvadasi

If, after a suddha ekadasi, a dvadasi tithi starts before the next sunrise and extends past the next sunrise after that, i.e. the dvadasi is vrddhi, the first dvadasi is called vyanjuli-mahadvadasi. On that day one should observe ekadasi vrata.

Trisprsa-mahadvadasi

If the ekadasi tithi presides at the dawn of a day and the dvadasi is completed before the next sunrise, i.e. the trayodasi tithi starts on the same day, that day where the three tithis touch is called trisprsa-mahadvadasi. One should fast on that day. Note: This may occasionally coincide with the unmilani rule above. If they do coincide, our convention is to just say unmilani.

Paksavardini-mahadvadasi

If a new or full moon day is vrddhi, crossing two consecutive sunrises, the preceding dvadasi is called paksavardini-mahadvadasi. One should observe the ekadasi vrata on that day.

Jaya-mahadvadasi

On a gaura dvadasi day where the dvadasi tithi extends until sunset, if punarvasu naksatra is present at sunrise and lasts until the next sunrise, it is called jaya-mahadvadasi.

Jayanti-mahadvadasi

On a gaura dvadasi day where the dvadasi tithi extends until sunset, if rohini naksatra is present at sunrise and lasts until the next sunrise, it is called jayanti-mahadvadasi.

Papa-nasini-mahadvadasi

On a gaura dvadasi day where the dvadasi tithi extends until sunset, if pusya naksatra is present at sunrise and lasts until the next sunrise, it is called papa-nasini-mahadvadasi.

Vijaya-mahadvadasi

On a gaura dvadasi day, if sravana naksatra is present at sunrise and lasts until the next sunrise, it is called vijaya-mahadvadasi.

Source
https://www.google.com/search?q=Mahadvadashi

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.

Source

http://www.iskcondesiretree.net/page/paksa-vardhini-mahadvadasi