In Indian mythology, Vaikuntha is the abode of the Vishnu. Ekadashi is eleventh day of the month. It is one of the most auspicious days, and is dedicated to Lord Vishnu. According to Vishnu Purana, if you fast on this one day, it is equivalent to fasting on all other 23 Ekadashis of the year. It is also believed that Door of the Vaikuntha opens on this auspicious day. Vaikuntha Ekadashi falls on Margashira / Margashirsha Shudha Ekadashi (Margashira masa Shukla paksha Ekadashi tithi).
The Bhagvadgeetopadesh from Krishna to Arjun is said to have done on this day.
Some Puranas say that it was on this day that the churning of the ocean occurred and the Amruta (nectar of immortality) emerged out of the ocean was distributed to Devas (gods).
According to Padma Purana, the female energy of Vishnu killed demon named Muran in the form of damsel named Ekadashi to protect devas.
The day is observed by complete or partial fast. The complete fast is with no food at all. Partial fast is by refraining from specific food like rice, grains, and specific vegetables like onions. Instead, only fruits, vegetables, milk, and milk products are consumed. Though the fast is observed as other Ekadashi fasts, the difference is some kind of sweet is prepared to offer Vishnu. Special pooja is performed with faith and devotion to get blessing from Vishnu.