Payovrata is the vrata or penance observed by Goddess Aditi to propitiate Lord Vishnu.
It is usually observed for 12 days, and during that time the devotee subsists on a diet of milk alone. 
In Hindu Mythology
A notable event is that of Sage Kashyap who asked his wife, Aditi, to use this method to please Lord Narayana so that a solution could be found for her son who was defeated by the Asuras. This penance finally resulted in the Vamana Avatar sending Mahabali to the netherworld.