Nagula Chavithi, also known as Naga Chaturthi, is observed on the fourth day of the waxing phase of moon in Kartik Month in Andhra Pradesh and parts of Karnataka. A popular story associated with the day known as Naga Chaturthi Katha is listened or read on the day. It must be noted that there are several versions of Nagula Chavithi story and this is one among them.
Manasa was the daughter of Brahma and she studied under Lord Shiva. She was called Manasa as she was born from the mind, or manas, of Brahma. She was so pious that she did intense tapas and got the darshan of Sri Hari Vishnu.
Manasa was married to Sage Jaratkaru. The couple had a son named Aasthika. Manasa and Sage Jaratkaru had split before the birth of Aasthika. Therefore Aasthika studied under Lord Brahma and Lord Shiva and became very famous and powerful sage.
Years later King Janamejaya performed Sarpa Yajna to kill snakes as Snake Takshak was responsible for the death of his father, Raja Parikshit.
As a result of the yajna, all the snakes fell into the yajna fire and died but Takshak did not appear.
Soon King Janamejaya found that Takshak was circling around the seat of Indra, king of Devas, in heaven and Indra was protecting him.
Immediately Janamejaya ordered the saints to chant mantras to bring Indra down to the yajna fire. Soon the seat of Indra started to fly and it came down on to the Yajna ground.
The Devas approached Lord Brahma to save Indra. Brahma suggested that only Manasa can help them. Manasa promised the Devas that her son Aasthika had the power to help them and he will rescue Indra.
As ordered by his mother, Aasthika saved Indra, Takshak and other snakes by stopping the yajna.
For saving them, the snakes gave Manasa the boon that whoever listening to her story or reciting her name will escape from snake bites and all types of Naga Doshas.