The beautiful state of Kerala was once ruled by an asura(demon) king Mahabali who was the grandson of Prahalad . The king was greatly loved and respected in his kingdom and was considered wise, judicious and extremely generous. Kerala witnessed a golden era during the reign of Mahabali and the people were extremely happy. His bravery and strength of character earned him the name Mahabali Chakravathy.
Though belonging to the asura(demon) dynasty, King Mahabali was a devout worshiper of Lord Vishnu. The Gods were threatened by the popularity of this asura king. They wanted to find a way to put an end to his reign and to reaffirm their own supremacy .Aditi , the mother of the Gods appealed to Lord Vishnu whom Mahabali worshipped, for help.
King Mahabali was famous for his generosity , Lord Vishnu disguised himself as a dwarf brahmin called Vamana. He approached king Mahabali just as he was finishing his morning prayers and begged for a piece of land. The generous king granted Vamana his wish and asked him how much land he wanted. Vamana asked for three paces of land. At once Shukracharya, the adviser to king Mahabali, sensed that the dwarf was no ordinary brahmin and warned the king. King Mahabali felt it is very unfit for a king to go back on his promise and asked the dwarf brahmin to chose his land.
As soon as the king granted the Brahmin his wish the brahmin increased himself to the size of cosmic proportions. With one step he covered the earth and with the other he covered the skies , next he asked the king for a place where he could keep his foot for the third pace. Knowing that the third step would destroy the earth, the great king with folded arms offered his head for the brahmin to place his foot. Lord Vishnu put his foot on King Mahabali's head and pushed him into patala(underworld). The king on seeing the brahmin's true identity asked for just one boon.
The king was so attached to his kingdom and its people that he requested that he be allowed to visit Kerala once a year.Greatly moved by the kind king's devotion for his people Lord Vishnu granted him the wish.
It is the day of visit of King Mahabali that is celebrated as Onam every year. It is a tribute to the great king's sacrifice .Elaborate preparations are made for the king's homecoming . The people of Kerala show in their celebrations how happy they are and also show how they wish their king well.