Mokshada Ekadashi is observed during the waxing phase of the moon in the month of November – December. The importance of this Ekadasi was narrated to Yudhishtira by Sri Krishna and is found in the Brahmanda Purana. Fasting. Observing this Ekadasi helps in destroying all the sins. In 2015, the date of Mokshada Ekadashi is December 21.Ekadasi fasting is dedicated to Bhagvan Srihari Vishnu and is observed twice in every Hindu month.
There is a popular belief that observing Mokshada Ekadasi will help in granting heaven to dead forefathers. Legend has it that King Vaikanasa, a popular ruler, once had a dream that his father was in hell. The King found himself helpless as he could not help his father with all his power and riches. Soon he took the advice of Parvata Muni, who had the knowledge of past, present and future.
Parvata Muni found that the father of King Vaikanasa had committed some sins and as a result he was suffering in hell. To help his father the Saint asked him to observe Moksada Ekadasi.
Soon the king, his wife and relatives observed Mokshada Ekadashi and saved the King’s father from hell.
All the normal rules associated with Ekadasi fasting is observed on the day.
The day is also observed as Gita Jayanti.
Source – http://www.hindu-blog.com/2008/11/moksada-ekadasi-mokshada-ekadashi.html