How the date is fixed
Nag Panchami falls on the fifth day of the bright half of the lunar month Shravana (Shukla Panchami). The date is the day on which that tithi is running at sunrise (the usual rule, called udaya tithi).
About
Nag Panchami falls on the fifth tithi of the bright half of Shravana, in July or August, at the height of the monsoon when snakes leave flooded burrows. The tithi at sunrise is taken; if Panchami spans two sunrises the first day is used. The Nagas are the deities of Panchami tithi in the panchang, and the Mahabharata's story of Janamejaya's serpent sacrifice is read on this day. Some regions, including parts of Gujarat and Maharashtra, keep a second Nag Panchami in the dark half.
How it is observed
Images of serpents are drawn or placed at the door and offered milk and flowers; digging the earth is avoided for the day.