Reason behind celebrating Nag Panchmi

It is believed that Krishna, a Hindu God had saved the lives of people from the harassment of Kaliya, the snake. It is believed that one day, when Krishna was still quite young, was playing by the side of river Yamuna and his ball got stuck in the branches of a tree that was just by the side of the river. While trying to get that ball, Krishna fell into the river. When Kaliya, the snake attacked him, he fought and after some time the snake understood that he was not an ordinary child. This was when he pleaded Krishna not to kill him and Krishna spared him by taking a promise that he will not harass the people anymore. Nag Panchmi is celebrated as the victory of Krishna on Kaliya, the most dangerous snake.