King Hima's son
King Hima had a son of whom astrologers foretold that he would die of a snake bite on the fourth day of his marriage. When the day came, his young bride would not let him sleep. She piled all her gold and silver ornaments and coins in a heap at the door of their room, lit lamps all around, and sat up through the night telling him stories and singing songs.
At midnight Yama, the god of death, came in the form of a serpent. But the blaze of the lamps on the gold and silver dazzled him, and he could not enter. He climbed onto the heap of ornaments and sat there all night, listening to the songs and stories. At dawn he quietly went away, and the prince lived.
So on Dhanteras night a lamp is lit for Yama at the door, facing south (Yama Deepam), and gold, silver and new utensils are bought and brought home.
Dhanvantari
When the gods and the asuras churned the ocean of milk, Dhanvantari, an avatar of Vishnu, rose from it on the Trayodashi of Kartika, carrying the pot of amrita and the knowledge of Ayurveda. He is the physician of the gods, and the day is also kept as Dhanvantari Jayanti and National Ayurveda Day, with prayers for health, since health is the first wealth.