Nagula Chavithi is celebrated twice a year during the months of Karthik and Sravan. 'Nagula' means snakes and 'Chavithi' is the fourth day after every New Moon or Full Moon day. The one in the month of Karthik is celebrated on the fourth day after Deepavali, which falls on a New Moon day. Serpents are worshipped with great devotion and religious fervor and women and children observe a fast then worship the Snake God. Dressed in their most festive clothing, they offer milk at the snake hills, or complete a ritual at home, by placing a picture of an idol or of a snake. If possible the seven-hooded cobras are worshipped on this day, with oleanders and special sweets called "chimmili" (a sweet made with sesame seeds and jaggery), and "chalimidi" (a sweet made with rice and jaggery) are offered to the deity.
It is interesting to note that snakes are accorded a place of worship in Hindu religion, and are given prime importance by the Hindu gods. Of the "saptha lokas" (seven worlds) of the universe referred to in the Hindu mythology, 'sarpa lokam' (world of snakes) is one. Even today, many orthodox Hindus do not kill snakes. If they happen to come across one, particularly a cobra, they pay obeisance and step aside. Some name their children snakes if they appear in their dreams. While snakes adorn Lord Siva's neck, Lord Vishnu made it his bed in his heavenly abode, on the celestial ocean of milk. The most common pictures of Vishni are ones in which he reclines majestically on coils of the cobra, with the open hood as a cover for his head.
Legend has it that Lord Siva swallowed a poison that emerged during "Samudra manthan" - the churning of the ocean by gods and demons in search of 'Amrutham' - the nectar of immortality. A snake was used as a rope in the process and a terrible poison (garalam) was one of the many things that emerged from it. It threatened to engulf the whole world, but Lord Siva, who retained it in his throat, without swallowing it, lest it kill him. The poison in his throat had one affect � it turned blue. Hence, he became known as "Neelakantha" or the 'blue-throated one'.