Naag Panchamee is celebrated on the 5th day of Krishn Paksh of Shraavan Maas. This day Naag (snakes) are worshipped. Black gram is soaked in water, and unboiled  milk and that black gram are taken to the Naag.

This story of Naag Panchamee comes in Mahaabhaarat:

Kadru and Vinataa were two sisters. Kadru had ten thousand Naag as her sons and Vinataa had two sons - Garud and Arun. Kadru, the mother of the Naag, once bet with her sister Vinataa about the color of the tail of Indra's horse. She asserted that the tail was black even though the horse was absolutely white. According to Vinataa however, the tail of the horse was as white as the rest of the horse body. Kadru said whosoever will lose will serve to other as her maid servant. Vinataa was very sure about the horse's tail so she didn't care.

On the certain day, Kadru and Vinataa stood outside to see their bet's result. They waited for Indra's horse to come in the sky. At the right time Indra's horse came and both saw that the horse's tail was black. Vinataa got defeated and she had no choice except to work as a maid servant of Kadru.

In fact Kadru deceived her sister. She asked her sons, the Naag, to coil around the white tail of the horse so that it appeared black. When they refused to participate in deceit, Kadru cursed them to be consumed by fire for disobeying her. Brahmaa however softened the curse and sent them to live in the netherworld. This happened on the fifth day in the month of Shraavan and because the lives of the Naag were spared on this day, it is considered to be auspicious day for snake worship. At last they had to obey their mother.