History or myth, concept of Onam is good:

A generous king known as Mahabali had ruled Kerala

People were in happiness and harmony then;

Pilfering, betrayal, hording etc were unknown

Years went by, heavenly people got annoyed
...more

The harvest festival of Kerala, Onam will be celebrated on 02/02/09 and preparations have already begun in Navi Mumbai. Legend says that it is celebrated to welcome King Mahabali, whose spirit is said to visit Kerala at the time of Onam which also marks the beginning of the Malayalam calendar. In Kerala, the carnival can last for anything from four to ten days. Locally that is not the case, one day seems adequate enough. On Onam, people wake us as early as 4 am and the day begins with getting their home ready. Elaborate prayers are recited and special pujas are performed by the senior family member. Lamps are lit up in front of the idols and all members of the house join in for the prayers. Since the festival is that of a harvest celebration, prayers are offered that the harvest in the coming year will be just as bountiful. After this many families opt to visit a nearby temple. Dressing up in traditional finery also plays an important role and often new clothes are given to every membe ...more

Honestly I hv know clue except that it is celebrated for 10 days
some one answered like this:
To pray the god to required to good guru. ( mahabali got guru of sukrachariyar ) and this nakshatira day God vamana conquered mahabali

It was celebrated in Tamilnadu also decades back. Why it was given up is not know2n.

Onam is celebrated in the state of Kerala, in southern India. It's the biggest festival of the year there. The most spectacular celebrations take place in Trivandrum, Thrissur, and Kottayam.

The state government celebrates Tourism Week in Kerala during Onam. Much of Kerala's culture is showcased during the festivities.