Papamocani Ekadasi, or Papamochini Ekadashi, is observed during the waning phase of moon in the month of March – April. In 2015, the date of Papamochini Ekadasi is March 16 (in South, north and eastern parts of India) and March 17 (in western parts of India). It is believed that observing Papamochini Ekadasi will help in the redemption of sins.Ekadasi fasting is dedicated to Lord Vishnu and is observed on the 11th day of waning and waxing phase of moon in traditional Hindu calendar.
The importance of Papamocani Ekadasi was mentioned to Yudhishtira by Lord Krishna and is found in the Bhavishyottara Purana. Legend has it that an Apasara named Manju Ghosha lured Sage Medhavi into married life. After several years, the Apasara wanted to return back to Devaloka but this was objected by the Sage. But the Apsara was unwilling to listen to the sage and in anger he cursed the Apsara and turned her into a witch. The sage told her that she will be relieved of the curse by observing Papamochini Ekadashi.
The sage too felt that he too had committed sins by falling into the trap of the Apsara and he too decided to observe Papamochini Ekadashi. Papa means ‘sin’ and ‘Mochini’ means to escape. Thus this Ekadashi exclusively helps in destroying sins and guilt associated with the sins committed.
All the normal rules associated with the Ekadasi fasting is observed during Papamochini Ekadasi
source – http://www.hindu-blog.com/2009/03/papamochini-ekadasi.html