Kamika Ekadasi is observed during the waning phase of moon in the month July or early August. In 2015, the date of Kamika Ekadasi is August 10. The importance of thisEkadashi was narrated by Lord Krishna to Yudhisthira and is found in the Brahmavaivarta Purana. Originally, it was narrated by Lord Brahma to Narada Maharishi. It is believed that observing this Ekadashi is equal to bathing in sacred rivers on auspicious days.
Offering Tulsi leaves to Lord Vishnu on this day is considered highly auspicious. Another important offering made is lighting ghee or sesame oil filled lamps at Sri Krishna Temples. Observing this Ekadasi is also equal to reading all the scriptures dedicated to Lord Vishnu.
It is believed that all the sins and remorse will be washed away if a devotee strictly observes the Kamika Ekadashi fast. It is mentioned in scriptures that those devotees fasting on this day wins the favor of Yamraj, the lord of death. For fasting the usual method is followed.
Source – http://www.hindu-blog.com/2008/07/kamika-ekadashi.html