Ananta Chaturdasi is believed to be the day when Lord Vishnu appeared in the form of Anantapadmanabha – the Ananta Sayana form of Lord Vishnu – reclining on Ananta. In 2012, the date of Ananta Chaturdasi is September 29. Ananta Chaturdashi is also the last day of the 12-day Ganesh Chaturthi festival.
Also known as Anandapadmanabha Vrata or Ananta Vrata, it falls on the fourteenth day after the new moon in the Hindu month of Bhadrapada.
In the Mahabharata, Shri Krishna advises Yudhishtara to observe Ananta Vrata for 14 years to get back his lost kingdom. A fast is observed from sunrise to sunset by Lord Vishnu devotees.
Special pujas are offered in temples dedicated to Lord Vishnu. Ananta Dharam a sacred thread of 14 knots is tied on hands after puja by devotees.
Today, Ananta Chaturdasi is popular with several devotees as the last day of Vinayaka Chaurthi festival – the day to perform the Ganesh Visarjan.
More details about Ananta Chaturdasi can be found in this post.