Chaturmas is the four months of the monsoon season in Hinduism in which strict austerity is observed and four kinds of rituals are performed. The four rituals are Vaisvadeva Parvan, Varuna Pra Ghasa Parvan, Saka Medha Parvan and Sunasiriya Parvan.
These rituals are formulated in Vedic texts and in Srauta Sutra. Though these rituals form a part of Rajsuya, they have separately formulated and they are performed independently. Apastamba proclaims that the punya (merit) obtained by the performance of these rituals is imperishable, and hence everlasting. These rituals are started in the first month.
In Vaisvadeva Parvan, eight deities are worshipped by making offerings and oblations. They are Agni, Soma, Savitr, Pusana, Saraswati, Maruti, Vishvadevah (group of gods) and Dyava Prithvi. Because Vishwadevah are worshipped as the main deity, this ritual gets the name Vaishvadeva Parvan.
The Varuna Pra Ghasa Parvan is performed in a twin altar ritual ground (north and south). In this ritual, Varuna (the water god) is worshipped as the main deity. Moreover, the leaves of a sacrificial tree are offered as ghasa (food) to the goat-shaped ake,meant for Varuna and hence this is known as Varuna Praghasa. At the end of the ritual, there is an avabhrita (holy bath) in which an offering is made to Varuna in water.
The ritual of Saka Medha Parvan is performed on two consecutive days. On the first day, three rituals are performed one each in the morning, at midday and at night. In the morning puja, Agni is worshipped. At midday and at night Maruti is worshiped. On the second day, five rituals are performed one after the other.
In Sunasiriya Parvan, ten deities are worshiped. The first five are the same as in Vaisvadeva Parvan. The remaining five are Indragni, Vishwadevah (a group of gods), Indra Sunasira (Lord of gods), Vayu and Surya. Here Indra Sunasira is the chief god and hence the ritual gets the name Sunasiriya. Sunasira means the presiding deity of the year and this place is attributed to Indra.
The smritis say that in each of the four rituals, hundreds of Brahmins are to be given food for the completion of the rituals performed.