Varuthini Ekadashi is observed during the waning phase of moon in March/April. In 2015, the date Varuthini Ekadasi is April 14 /15. It is believed that observing Varuthini Ekadasi will make a person fortunate, prosperous and helps in redemption of sins and in attaining moksha. The greatness of this Ekadasi fasting was narrated to Yudhishtira (the eldest of Pandavas) by Lord Krishna and is mentioned in the Bhavishyottara Purana.
It is believed that by observing Varuthini Ekadashi one can attain the result of performing different types of charities. All the regular fasting rules attached to Ekadasi areobserved during Varuthini Ekadasi.
Ekadasi fasting is dedicated to Lord Vishnu and is observed on the 11th day of waning and waxing phase of moon in a traditional Hindu calendar
Source – http://www.hindu-blog.com/2009/04/varuthini-ekadasi.html