There lived an ASURA king by the name Mahabali. Though he was an ASURA, he was a good and great king loved by his people. He looked after his kingdom and people very well. People lived with equality and there was no room for discrimination. Hence the land where he ruled became more beautiful and prosperous than heaven.
Now the spineless Devas were jealous because no people bothered to care about them or even worshipped them. So the Devas went to Lord Vishnu and sort for help. Vishnu took the avatar of Vamana and went to Mahabali. Vamana asked for 3 feet of land from Mahabali. Mahabali, being generous granted the wish. Then, Vamana became an enormous giant and took the other world and earth in two feet. As there was no other land, Mahabali told Vamana to place his feet on his head. Trickster Vamana did so and sent Mahabali to underworld. Before leaving to the underworld Mahabali asked for one wish from Vamana- that he may visit his land ‘Kerala’ once a year. Vamana agreed.
So the day Mahabali returns to visit his land, we celebrate Onam – with grand celebrations.
Now the question – Why do we celebrate Onam?
Mahabali was an ASURA king and ASURA’s are bad according to so called Devas. But Mahabali was a great king, loved by his people. Unlike other festivals where Devas conquers Asuras, Onam is quite different. Here a good man is defeated for being too good to his people. Onam is a celebration for the return of that good King. Onam is a reminder that no matter what your origins are, if you are good to your people and loved by your people, you will be remembered and you will be worshipped.