Vata poornima fasting is observed by married women for the well being of their husbands in certain parts of India. Vat Purnima 2011 date is June 15. The fast is based on Savitri Satyavan legend in the Mahabharata. The Vrat is observed during Poornima (Full moon) in the month of Jyeshtha and is more popular in Gujarat and Maharashtra. In some regions it is a three-day Vrat.

For those people who observe the traditional three-day Vrat in India, Vat Vat Purnima Arambh (Begins) on June 13, 2011 and ends on June 15.
Prayers and pujas are dedicated to Vat Vriksha (Banyan Tree) and Savitri. Most people now fast only on the main ritual day that is on the Poornima day. The ritual is also known as Peepal Pujan.
The main ritual on the day includes tying of the red or yellow thread around the banyan tree and fasting from sunrise to sunset.

Vat Savitri Puja or Vat Savithri vrat is an auspicious day in Hinduism when married women observe fast and pray for their husband's health and longevity. The fasting is named after Savitri, who brought back her husband from the clutches of Yama (death). One of the important rituals of the day is tying threads on Vat (banyan) tree. In 2011, the date of Vat Savitri Purnima puja is on June 15. Those observing it on Amavasi day it is on June 1 - Vat Amavasi.
The fasting is observed for three days and begins two days prior to purnima or amavasi in the Hindu month of Jyeshtha (June – July).
The fasting takes its name from Vat Vriksha (Banyan Tree) and Savitri. The Banayan Tree is symbolically represented as Brahma, Vishnu and Shiva. The root of Vat Vriksha is Brahma, the stem is Vishnu and the upper part is Shiva. On the puja day, the Banyan tree also symbolically represents Savitri and the incident mentioned in the Mahabharata.
Legend has it that Princess Savitri, the daughter of King Ashwapati of Bhadra Kingdom, fell in love with Satyavan, a woodcutter. But Satyavan was destined to die within a year and Savitri was made aware of this fact by Rishi Narada. But Savitri decided to get married to Satyavan and live with him in the forest.
As predicted, Satyavan fell from a tree and died within a year. Yamraj, the god of death, arrived to carry him away. Savitri made it clear to Yamraj that she will follow Yamraj along with her husband. Yamraj tried several ways to deter Savitri from following him but all his efforts were in vain and Savitri remained adamant.
Finally, Yamraj was moved by Savitri’s devotion and he brought back Satyavan to life.
It is believed that Satyavan spend his last moments under a Vat or Banyan tree on the full moon day in month of Jyeshtha. And Yamraj appeared here and Savitri pleaded with Yamraj under the Banyan tree. In memory of this event, women go round the Banyan tree for 108 times tying threads and fast for the health and longevity of their husbands.