(If Mahadvadasi is in force, fasting is prohibited on the Ekadasi, but the observance is done on the Dwadasi. Compound Dwadasi-Mahadwadasi.)
There are eight special conditions when a dvadasi lunar tithi becomes a Maha-dwadasi.
In such cases, fasting is done on the Maha-dwadasi and not the ekadasi day.
NOTE: A Maha-dwadasi may be in effect for one location and not another.
A regular dwadasi can also become a fast day for an ekadasi under certain conditions.
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.
Note::: Special thanks to Jaya Tirtha Caran prabhu from NZ for allowing us to use some of the content from his site to compile these pages.