Each month consists of 29 to 30 days, and is based on the phases of the moon. Each month is divided into two fortnights (paksha). Shukla Paksha (Sudi) ends with a Poornimaa (full moon), and Krushna Paksha (Vaadi) ends with a Amaavasyaa (New Moon). In some regions, (eg., Uttar Pradesh) the month starts with Krushna Paksha following Poornimaa, but for most regions of Bharat, the month starts with Shukla Paksha following Amaavasya.
The twelve months of the lunar year correspond to the following calender months and make up the six seasons (Ritu) :
2.Vaishakh (April-May) ............... Vasanta Ritu (Spring)
4.Aashaadh (June-July) ............... Greeshma (Summer)
6.Bhadrapad (August-September) ......... Varsha (Monsoon)
8.Kaartik (October-November) ......... Sharad (Autumn)
10.Paush (December-January) ......... Hemanta (Winter)
12.Phalgun (February-March) ........... Shishira (Dewey)
Adhika (Purushottam) Maas
Since the calendar is based on the phases of the moon, the twelve as above take 354 days, 8 hours and 34.28 seconds. This creates a difference of 10 days, 21 hours and 35.16 seconds from the actual solar year (365 days, 6 hours, 9.54 seconds)
When the accumulated difference exceeds 29 days, 12 hours, 44 minutes and 2.865 seconds, an adjustment is made with a extra month (Adhika Maas), which carries the name of the previous or the next month, depending on the proximity of the month. Normally, seven extra months occur in 19 years.