In this calendar, the basic unit was a thithi or lunar day. Approximately 30 of these made a Lunar month. This was about 29 and a half solar days. This month was divided into 2 halves of fifteen thithis each. Each half is called paksa. The paksa beginning with full moon is called purnimavasya and other beginning with new moon is called amavasya. The fortnight beginning with new moon is called bright half or shulkapaksha and dark half is called krishnapaksha. In most of North India, the new month began with full moon.
The year normally contained 12 lunar months. These are given below:
Chaitra (March-April), Vaisakha (April-May), Jyaistha (May-June), Asadha (June-July), Shrawan (July-August), Bhadrapada (August-September), Ashwin (September-October), Karttika (October-November), Margashirsa or Agrahayana (November-December), Pausa (December-January), Magha (January-February) and Phalguna (February-March). The year began with Chaitra.
In earlier times the names of these months were: Madhu, Maadhva, Shukra, Shuchi, Nabhas, Nabhasya, Isha, Urja, Sahas, Sahasya, Tapas and Tapasya. These names were used in poetry.
The lunar year was thus 354 days long thus creating a discrepancy between lunar and solar years. 62 lunar months are approximately 60 solar months. So every 30 months an extra month was added generally inserted after Asadha or Shrawana and called second (dwitiya) Asadha or Shrawana.
A group of every two months formed a season or ritu. The six seasons are Basanta (Spring, March-May), Grishma (Summer, May-July), Varsha (The Rains, July-September), Sharad (Autumn, September-November), Hemanta (Winter, November-January) & Shishira (Cool Season, January-March), This shows how in comparison to European or American continents, Weather in Indian continent changes throughout the year. This gives rise to a variety of crops, fruits and uncertainties and literature.