Adhik Maas means Extra Month is also known as Purushottam Mass, Mal Maas, Malimmacha. We have two calendars in India. One is Lunar also known as Hindu calander and the other is Solar. A lunar month is around 29.5 days long and a solar month, that is the usual calendar, is 30 to 31 days. The lunar year consists of 354 solar days and solar year consists of 365 days. There is a difference of 11 days between the lunar and solar year. As this gap increases each year, it approximates in three years to one month. To compensate and get the two calendars similar an extra month is added. This is the thirteenth month of the lunar calendar.
Vasishta Siddhanta, a treatise of Sage Vasishta, says that the Adhika Maas occurs after every 32 months, 16 days and 8 Ghadis. A Ghadi is 24 minutes.
The Adhik Maas is added when the Amvasya and the Surya Sankranti fall on same day. For example, on 14th April 2010, it is Amvasya and also the Sun is changing its sign from Pisces to Aries. So, there is an Adhik Maas this year. This is the ongoing month of Vaishakh which comes after the month of Chaitra. Vaishakh began on 30th of March 2010 and will end on 30th of May 2010 lasting for two months instead of the usual one month.
Just as there is the lunar year with the extra month (Adhik Maas), so is there a lunar year with a diminished or reduced month (Kshaya Maas), with only eleven months in the year. The lunar year comprising of eleven months only is very rare indeed. But the extra month or Adhik Maas comes every third year. Adhik Maas adopts the name of the month that follows Adhik Maas. Whenever Sankranti does not fall within one month then Adhik Maas occurs. When there are two Sankrantis within one month then it becomes the lunar year with one reduced month.
This year Adhika Maas is from March 30, 2010 to April 28, 2010 as per North Indian Calendar, for whom the new month starts on the full moon. In Gujarat, Maharashtra, Karnataka and Andhra Pradesh, the Adika Maas is from April 15, 2010 to May 14, 2010, as the new month for these people start from the new moon. This Adhik Maas is dedicated to Lord Krishna and as Vaishak is auspicious to Vishnu this makes the Adhik Maas in 2010 doubly auspicious.