The concept of Adhika Masa is unique to the traditional Hindu lunar calendars. In 2015 Hindu calendar, there is an Adhika Masam or Adhik Masa or an extra month. This Adhika Masam is from June 17, 2015 to July 16, 2015 as per North Indian Calendar. In Gujarat, Maharashtra, Karnataka and Andhra Pradesh, the Adika Masa is also from June 17, 2015 to July 16, 2015. This extra month is known as Adhik Ashada Maas or Adhik Ashad Mahina. The Adhika Masa is dedicated to Sri Krishna.
So in 2015 there are thirteen months in Hindu Lunar calendar. Apart from the normal 12 months, there is an an Adhik Ashada Maas.
It must be noted here that there is no Adhika Masam in the calendars followed in Assam, Bengal, Kerala, and Tamil Nadu as it is based on Solar Calendar.
Why There is an Adhika Masa?
The Hindu calendar is a lunar calendar and is based on the cycles of the Moon. A lunar month is around 29.5 days long. Now, 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. So, as years pass by each lunar month starts earlier to the corresponding solar month. There is a difference of 11 days between the lunar and solar year. In every two or three solar years the difference between the two calendars becomes a full month or around 29 days. To compensate and get the two calendars similar an extra month is added
Vasishta Siddhanta, a treatise of Sage Vasishta, says that the Adhika Masa occurs after every 32 months, 16 days and 8 Ghadis. A Ghadi is 24 minutes.
Now, in Andhra Pradesh, Karnataka, Maharashtra andGujarat the lunar month begins and ends with the new moon or no moon (amavasya). In North India, the lunar month begins and ends with full moon (purnima).
How is Adhika Masa Calculated?
The sun changes its sign or rashi (like Mesha, Vrishabha) every month in Hindu astrology system. The day it enters a Rashi is called as its ingress into a Rashi. A lunar month in which there is no sun ingress into a Rashi is considered an Adhika Masa. Adhika Masa occurs once in 32 solar months and 16 days.
For an average devotee, the Adhika Masa means some change in the temple celebrations in the States following the lunar calendar. For example, due to the Adhika Masam, there will be two Brahmotsavams in Tirupati this year. And the Adhika Masa is dedicated to Lord Krishna.