The Story of Nag Panchmi festival
A temple opens on this day only in Mahakaleshwar shrine
Nagchandreshwar darshan happens in a year only on the fifth day of the Shukla Paksha of Shravan month.The temple opens its doors annually only on Nag Panchmi.The temple will open after proper worship and religious ceremonies to facilitate devotees to pay obeisance to the deity.
Astika saves Nagas
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 Manasa devi 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.
At his mother's request, Astika went to the sacrifice, and obtained a boon from Janamejaya that the snake-sacrifice should be abandoned mid way. Thus he fulfilled the purpose of his birth, saving his kinsmen, the snakes from utter destruction.
Other legends of Nag Panchmi
Sri Padmanabhaswamy Temple Mahavishnu Idol
There is also a mythological story about a goddess Sathyeshwari, whose brother Sathyeshwar died before the day of Nag Panchami. She grieved over the death of her brother without eating anything. She saw her brother in the form of a cobra and believed that it was her brother. So, Nagdev promised her that he would protect any woman who deems a cobra as her brother and worships it. Hence, it became the habit of Hindu women to worship snakes for the longevity and safety of their brothers on this day.
Story of serpent and farmer
In the context of Nag Panchami, many stories are famous. As per one of the story, a farmer used to live in a state with his two sons and one daughter. One day while ploughing the field, three children of snakes came under it and died. After the death of Naag(male snake), at first, Naagin(female snake) expressed sorrow by mourning, then, planed to take the revenge from the murderer of its sons.
In the darkness of night, Naagin, killed the farmer, his wife and his two sons by biting them. Next day morning, Naagin(she-serpent) came to bite the farmers daughter. The girl kept a bowl full of milk and joined hands for forgiveness,in front of Naagin. By this gesture of the girl, Naagin got happy and gave back the lives of farmer, his wife and two sons.
That day it was,Panchami Tithi of Shukla Paksha in Shravan Maas. From that day, to be safe of Serpent's anger, they are worshiped on this day and Naag Panchami is celebrated.