Onam which is common name for Thiruonam is celebrated on September 16th 2013.
Onam marks the visiting of the legendary King Bali to Kerala, which was once his kingdom. Bali was forced to PatalaLoka. It was Lord Vishnu in the form Vamana who forced King Bali to move to Patala Loka. As an acknowledgement of his sacrifice Lord Vishnu granted him a boon of returning to his kingdom once an year.
The day on which King Bali returns to his kingdom is celebrated as Onam. The festival begins 10 days before itself.
The following table shows all the festivals as a part of Onam: