Hindu calendar, based on the movement of the moon, has twelve months. According to the Western calendar, a year is the time it takes for the earth to complete one orbit around the sun. But, in the Hindu calendar, a solar year is divided into 12 lunar months. Each month consists of 29 to 30 days, and is based on the phases of the moon.
Each month has two fortnights, bright half (the period from a new moon to full moon) and a dark half (the period between a full moon and the next moon). The lunar days in the Indian calendar are called tithis, which are calculated using the difference of the longitudinal angle between the position of the sun and moon.
The twelve months of the lunar year are Chaitra (March-April), Vaisakha (April-May), Jyestha (May-June), Ashada (June-July), Shravan (July-August), Bhadrapad (August-September), Ashwin (September-October), Kartik (October-November), Margasheersh (November-December), Paush (December-January), Maagh (January-February) and Phalgun (February-March).
The twelve months as above take 354 days, which create a difference of 10 days from the actual solar year (365 days). Hence, an adjustment is made with an extra month Adhika Maas, which bears the name of the previous or the next month. Usually, seven extra months occur in 19 years.