On Nag-Panchami, snakes are worshiped and offered milk to drink.

But, Snakes are reptiles, not mammals so they cannot drink milk Milk cannot be digested by reptiles and they eventually die!

The Reason why snakes drink milk on Nag-Panchami is because they are dehydrated and stressed 30-45 days before Nag Panchami by the Snake Charmers.

source: http://www.arpfindia.org/blog/truth-behind-nag-panchami