The festival of Onam falls in the Malayalam month of Chaingam i.e. during the month of the August-September according to the Gregorian calendar. During this time a number of the events take place throughout the state as the snake boat races, flower carpet weaving, the Onappottan etc. The people take pride to take part in the festivals and clad themselves in new clothes and a good number of traditional dishes get prepared in the homes during this ten day celebration.
As the legend goes here in Kerala and as mentioned in the ancient Indian scriptures, Onam has been usually celebrated as a welcome of the asura king of the ancient Kerala king Mahabali. Though he was from asura tribe, the protagonists of the heaven gods, he was known to be a very kind ruler and the people of his kingdom were very much happy and prosperous during his reign. It is said that to check the increasing ego of lord Indra who was the king of heaven, lord Vishnu bestowed King Mahabali with powers that made him very much powerful and popular among the masses due to his kindness and the justice. This led to the popularity of the king among the people and the gods and thus shaking the ego of the gods of heaven. Lord Indra learnt a lesson and to check the increasing power of the king of the Kings Emperor Mahabali all the gods pleaded to lord Vishnu.
Lord Vishnu had to incarnate as a dwarf Brahmin and his incarnation is called Vamana avatar. He took advantage of the generous nature of the king and asked for a very small gift from Mahabali which was just three feet of land. King Mahabali agreed to give it but lord Vishnu in his Vamama Avatar took a very huge form so that at his two feet covered the all the land and the heavens and on asking where to keep the third feet Mahabali offered his head on which lord Vishnu stepped to send him to the Netherworld.
After that time it is believed that the beloved king of the people of the Kerala visits them during the month of Chaingum and the people welcome their beloved king with plenty of love and affection and thus remembering his prosperous rule. The times are filled with plenty of feasts and the people sing the welcome songs for the king called "Maveli nadu vaneedum kalam" during this time.