
People who knew about onam often wonder why this festival is only celebrated in Kerala. onam is though celebrated as a harvest festival in kerala there is also a historical story behind it. Asura king Mahabali, who reigned kerala, planned to perform Mahayajna, which was safeguarded by Lord Shiva, prompting the Devas to seek Lord Vishnu's help in breaking the Yajna in an attempt to prevent Mahabali from seizing Indra's throne. Lord Vishnu, therefore, took the avatar of Vamana to approach the Yagnaja. Bali Chakravarti who was a great "Dhaana Dharmisht" was approached by vishnu in Vaman avatar and was asked for three feet of land. Which Mahabali consented to.
But Vamanan on the other hand grew like a giant and placed his one foot on the heavens and the other on the entire universe leaving no room for the third step, Mahabali offered his head for the third foot and was taken to the Pathallokh (underworld) vishnu bestowed him a wish to visit his people at this time every year and hence his return is widely celebrated as onam in Kerala.