There are essentially two types of traditional Indian calendars. One is the solar calendar (Surya ganana) based on the transit of the sun and the other is the lunar calendar (Chandra ganana) based on the transit of the moon. Both are based on the niryana system of calculating the planetary positions.
Solar Calendar:
The names of the 12 months are Chaitra, Visakha, Jyestha, Ashadha, Shravana, Bhadrapada, Ashwija, Kartika, Margasira, Pushya, Magha and Phalguna.
The Solar Calendar is based on the Sun’s entry into various signs. For example the Sun’s entry into the zodiac sign Aries, usually on April 13/14 th, starts the month Vaisakha. Similarly its entry into the sign Capricorn, usually on January 14/15 th is the start of the month Magha. The starting of the year varies from culture to culture. In most parts of India the “day” of Sun’s entry into a sign is based on sunrise to sunset. If after sunrise it is taken as the next day. In Bengal and Assam it is before midnight or 45 ghatis.Indian National Calendar: The official Indian calendar was was introduced by the Govt. of India from 22nd March 1957, as per the Calendar Reform Committee. It is based on the Sayana system Sun’s entry into the signs. The following are the starting days of each month.
Chaitra March 22
Vaisakha April 21
Jyaistha May 22
Ashadha June 22
Shravana July 23
Bhadrapada Aug 23
Ashwija Sep 23
Kartika Oct 23
Margasira Nov 22
Pushya Dec 22
Magha Jan 21
Phalguna Feb 20