Thiruonam. The most important festival of Malayalis, celebrated across Kerala irrespective of religion or caste.
Most of the major festivals in India have more than one story associated with them. Onam is one of the festivals which have only one legend describing it. The banishment of the good King Mahabali of Kerala by Lord Vishnu and his arrival every year to see his citizens. Gods are sometimes unfair to man, in the name of fulfilment of their dharma.
Not a Malayali? Read this website for the story of Onam.