Kamada Ekadashi is observed during the waxing phase of moon in March/April. It is said in the scriptures that observing Kamada Ekadasi will help in getting rid of sins and increasing virtue. The greatness of this Ekadasi fastingwas narrated to Yudhishtira by Lord Krishna and is mentioned in the Varaha Purana. In 2015, the date of Kamada Ekadashi is March 30.
Legend has it that a Gandharva was unable to perform his duties when his wife was not around. This Gandharva was overpowered by lust and thus failed in his duties and was cursed by a King to become a cannibal. The Gandharva and his wife roamed the forests and one day the wife came across a Sage, who advised them to undertake Kamada Ekadashi fasting for getting rid of the curse.
The Gandharva couple observed the Kamada Ekadashi and the husband was freed from the curse.
On the Ekadashi day, most Vishnu devotees fast and some devotees keep vigil at night. Those people undertaking partial fasting abstain from food made of rice.
source – http://www.hindu-blog.com/2009/04/kamada-ekadashi.html