Mohini Ekadashi occurs during the Shukla Paksha (waxing phase of moon) in the month of April/May. In 2015, the date of Mohini Ekadasi is April 29. Ekadasi fasting, which falls on the 11th day of a lunar fortnight, is dedicated to Lord Vishnu. It is believed that one will be able to overcome sadness and remorse by observing Mohini Ekadasi.

Mohini Ekadashi falls in the month of Vaishakha in calendars followed in North India and other regions. It falls during Chithirai Month in Tamil Calendar.

Importance of Mohini Ekadashi is mentioned in the Surya Purana. The greatness of it was narrated to Yudhishtira by Lord Krishna.

It is said that Lord Vashishta advised Lord Ram to observe Mohini Ekadashi to get over the remorse and distress caused from the separation from Mata Sita.

All the regular practices associated with Ekadasi are followed during Mohini Ekadasi. Those observing partial fast avoid rice.

source – http://www.hindu-blog.com/2009/05/mohini-ekadashi.html