Kokila Sashti is observed in Vaisakh month and is dedicated to Goddess Sati. Kokila Sashti 2017 date is April 17. Shashti or Shasti is the sixth day during a lunar fortnight.
There is a legend that Goddess Sati spent 1000 celestial years as a cuckoo before she got back her shape and merged with Shiva. This happened after the incident mentioned in the Puranas in which Sati immolated herself.
Kokila Sashti is observed during the Vaishakh Krishna Paksha or waning phase of moon.