Kokila Vrat is observed on the full moon day in the lunar month of Ashadha. It is said that when there is an intercalary Ashadha month in a particular year, then Kokila Vrat should be observed in that year. That means, Kokila Vrat should be observed only when Ashadha Month is leaped. This is applicable during normal Masa and not during Adhik Masa. This particular belief is followed specifically in North Indian states. In some regions of the Southern and Western India, Kokila Vrat is performed on Ashadha Purnima.
Kokila Vrat is dedicated to Goddess Sati and Lord Shiva and there is a popular legend behind it. Kokila refers to Indian bird cuckoo and it is associated with Goddess Sati. Goddess Sati sacrificed herself when her father Daksha insulted her husband Lord Shiva. Later, Goddess Sati spent 1000 celestial years in the form of cuckoo before she regained her shape and merged with Lord Shiva.
Kokila Vrat is usually observed by women and in some places this Vrat is observed for a period of one month from Ashadha Purnima to Shravan Purnima. During Kokila vrat, women take bath in the early morning and they make an idol of cuckoo with clay and start worshipping it.
The belief is that women who observe Kokila Vrat will become Akhand Saubhagyavati, which means they will never go through widowhood phase in their life, because they will always die before her husband. Another belief is that worshiping a cuckoo idol made of clay during Kokila Vrat will help them in getting a loving and caring husband