Apara Ekadasi occurs during the waning phase of moon in the month of Jyeshtha (May – June). The importance of Apara Ekadashi was explained by Lord Krishna to Yudishitira, the eldest of the Pandavas. Washing away the sins committed is the main benefit from observing Apara Ekadasi. In 2015, the date of Apara Ekadashi is May 14.
Fasting on this Ekadashi is beneficial to those people who are suffering from remorse or guilt due to sin or sins committed. One can attain moksha by observing a fast on Apara Ekadasi. Krishna also mentions that one who observes this particular Ekadashi will become famous through meritorious deeds.
Observing this Ekadasi removes the remorse associated with not performing a particular action, when the person had the opportunity to do it but shied away.
source – http://www.hindu-blog.com/2008/05/apara-ekadasi.html