The Mahasivarathri is essentially a religious festival unlike the Onam and Thiruvathira. The word means 'the great night of Siva'. According to the Sivapurana, it falls on the Krishna Chathurdasi day which is on the fourteenth day during the waning of the moon in the month of Megha, though in some years it may occur in Phalguna also. In Kerala the month of Kumbham is noted for the Sivarathri festival which falls in February-March.

The festival is said to commemorate the day on which Siva protected the world from a total annihilation either by drinking the deadly Kaalakoodum poison which was help up in his neck, or by effecting a healthy compromise between Brahma, the Creator and Vishnu, the Protector.

The Mahashivarathri is considered a very important day for fast and Siva worship. Though the observance offer Bali (sacrifice) to their ancestors in the morning succeeding the holy night.

In many respects the Sivarathri festival in Kerala can be considered a miniature Ardha Kumbha Mela held at Thriveni, the confluence of the holy rivers Ganga-Yamuna, and the invisible Saraswathi.