the story of Onam is that there was a king who owned the whole world and was very powerful but also nice and the gods got jealous of him. so Vishnu (one of the gods) came down from where the gods live in dwarf form and asked the king if he could have three steps of land. the king was warned by his sage not to but he gave the three steps of land anyway. so Vishnu grew to giant size an took two huge steps and he was across the whole earth. since the king promised Vishnu three steps and he was out of land to give he said that Vishnu could step on his head. all the kings people loved him and he was so noble in letting his head get crushed Vishnu let him come back to earth once a year to visit his people and that day is Onam.
onam is a holiday here in India