Vat Purnima (and its close variant Vat Savitri) is a fast observed by married women in Maharashtra, Gujarat and other western and northern regions for the long life of their husbands, kept, in the western reckoning, on the full moon (purnima) of the month of Jyeshtha (May-June). Its emblem is the banyan (vata) tree, long-lived and far-spreading, an image of the enduring married life the fast prays for: women gather at a banyan, worship it, and wind a thread around its trunk while circling it, offering water and prayers.
The observance rests on the story of Savitri, one of the most admired heroines of the Mahabharata: warned that her chosen husband Satyavan is fated to die within a year, she marries him nonetheless, and when Yama, the god of death, comes to take his soul, she follows and so wins him with her wisdom and devotion in argument that Yama, granting her boons, is maneuvered into restoring her husband to life. The banyan is the tree under which, in the tale, Satyavan died and was revived. Like Karva Chauth, to which it is closely comparable, Vat Purnima is a women's vrata centered on conjugal devotion, holding up Savitri's steadfastness and cleverness as the model of the ideal wife.