The main reason of celebrating this day must be that snakes are a great threat to mankind during these months.They usually come out of their holes as rainwater seeps in and while looking for shelter they might harm humans. However, this is why they are worshiped this day and fed with milk. 

Mahabharata epic, Janamejeya, the son of King Parikshit of Kuru dynasty was performing a snake sacrifice known as Sarpa Satra, to avenge for the death of his father from a snake bite by the snake king called Taksaka. A sacrificial fireplace had been specially erected and the fire sacrifice to kill all snakes in the world was started by a galaxy of learned Brahmin sages. The sacrifice performed in the presence of Janamejaya was so powerful that it was causing all snakes to fall into the Yagna kunda (sacrificial fire pit). When the priests found that only Takshaka who had bitten and killed Parisksihit had escaped to the nether world of Indra seeking his protection, the sages increased the tempo of reciting the mantras (spells) to drag Takshaka and also Indra to the sacrificial fire. Takshaka had coiled himself around Indra’s cot but the force of the sacrificial yagna was so powerful that even Indra along with Takshaka were dragged towards the fire. This scared the gods who then appealed to Manasadevi to intervene and resolve the crisis. She then requested her son Astika to go to the site of the yagna and appeal to Janamejaya to stop the Sarpa Satra yagna. Astika impressed Janamejaya with his knowledge of all theSastras (scriptures) who granted him to seek a boon. It was then that Astika requested Janamejeya to stop the Sarpa Satra. Since the king was never known to refuse a boon given to a Brahmin, he relented, in spite of protects by the rishis performing the yagna. The yagna was then stopped and thus the life of Indra and Takshaka and his other serpent race were spared. This day, according to the Hindu Calendar, happened to be Nadivardhini Panchami (fifth day of bright fortnight of the lunar month of Shravan during the monsoon season) and since then the day is a festival day of the Nagas as their life was spared on this day. Indra also went to Manasadevi and worshiped her.