Mahadwadasi is an important day associated with Vaishnavas
and those observing Ekadasi Fasting. If majority of the Ekadasi fasting is
observed on the Dwadasi day (12th day) instead of the 11th
day then this Dwadasi is referred to as Mahadwadasi.
Such Ekadasi fasting is observed on the Dwadasi and is
broken on the 13th day day (trayodasi tithi).
Mahadwadasi happens a couple of times in a Hindu lunar
calendar. There is no other specialty attached to the day.