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Onam (Malayalam: ഓണം) is the biggest festival in the Indian state of Kerala. It falls during the first month of the Malayalam calendar which is Chingam (August–September) and marks the homecoming of the legendary King Mahabali. The festival lasts for ten days and is linked to many elements of Kerala's culture and tradition. Intricate flower carpets, elaborate banquet lunch, snake boat races, Puli Kali, and the kaikottikkali dance all play a part in the festival.

extract from:-
http://en.wikipedia.org/wiki/Onam