Its a story of selfless sacrifice, charity and love for the citizens of Mahabali's Kingdom.It brings out best in a man against all odds and personal loss.
Since his Kingdom was handed over to Vamana( one of the 10 incarnations of Lord Vishnu), hence Kerala is appropriately called GOD's own Country.
The story behind ONAM is part of Pan Indian Culture. Hence I would greet all Indians a very happy ONAM.