Onam is the biggest festival of the people of Kerala which is celebrated with great enthusiasm, lasting for ten days, and the most important day is Thiru Onam. It falls in the beginning of the first month of Malayalam (Chingam). Onam is the celebration of annual home coming of the legendary king Mahabali who ruled this land many years ago. It is believed that during Mahabali's time, it was Kerala's golden era where all people were very prosperous and lived extremely happy, loving each other, without any negative issues. Incidentally, it is also the harvest festival.
Hindus also believe that it was this day when Lord Vishnu took His fifth Avataram as Vamana.
You can also check out my hub on Onam. http://lex123.hubpages.com/hub/Kerala-Festival-Ona...
If you want to know further please check out http://en.wikipedia.org/wiki/Onam