Significance of the festival and offering milk
Snakes occupy a very significant place in Hindu Mythology. Considered to be the resident of Nag Loka, snakes on Nag Panchami are worshiped for the protection of the family from evil energies. Number of myths and legends are associated with the festival.
One of the myth states that a deadly snake Kalia had been poisoning the river Yamuna making it difficult for the brijwasis (residents of Brij, Uttar Pradesh) to drink water. Krishna (an avatar of Vishnu) then takes down Kalia and defeats him. Kalia is forced to take back all the poison from the river, and in return Lord Krishna blesses him saying whosoever will offer milk and prayers to the snakes on this day (Nag Panchami) would be relieved of all his troubles and sins.
Another common myth associated with the festival goes back to the legend of the great Samudra-Manthan episode. During the epic episode in Hindu Mythology, the milk-ocean was churned where all kinds of herbs and potions produced were to be divided between Devas and Asuras. But the churning also threw a pot of deadly poison Kalakatoom, which had the power to wipe out entire creation. Lord Shiva drank all the poison that came out from the churning, giving him the name Neelkantha (the one with blue throat). In the whole process, a few drops fell on the ground as well, which were drunk by Shiva's closest aides - snakes. The toxic level of the poison was enormous, hence to calm down its impact, the Devtas performed Ganga Abhishek on Lord Shiva and the snakes. The act of offering milk is a symbolic representation of the episode.
Hindus also offer milk to snakes on the auspicious day to negate their Kal Sarpa dosh- an astrological imbalance in the position of their planets caused by Rahu and Ketu. Offering milk and rice to snakes and the Shivalingam are said to relieve one from all the calamity that lies ahead.
One can also find Lord Shiva adorning a snake on his neck in scriptures. The three coils around Shiva is the indication of the past, present and the future. Lord Vishnu is also represented as reclining on the bed of SheshNag, a five hooded snake in his characteristic calm and meditative stance.
Here's wishing you all a Happy Nag Panchami 2017!