Hindu calendar is Lunar calendar, and moon takes about 29.53 days to revolve around the earth. This makes lunar month 29.53059 days only. Adding all these 12 lunar months the year will have only 354.36708 days while Solar year is about 365 days. To adjust lunar months to solar year, Lunar calendar adds one extra month every third year.
This extra month is known by various names: Adhik Maas, Mal Maas, Purushottam Maas and Malimmacha. This is the thirteenth month of the lunar calendar. Therefore lunar calendar has 13 months in every 3 years.
The exact calculation is: this extra lunar month occurs after every 32 months, 16 days and 8 Ghadis (a Ghadi is a period of 24 minutes and 60 Ghadis equal 24 hours).
This year pursottam mahina starts form August 18th and ends on September 16 2012. This corresponds to Bhadra 2 to Ashwin 31, 2069 in Bikram Sambat Calendar.
Like Lunar year has 13 months, there comes also lunar year of 11 months only. It happens every 140 years to 190 years.
Adhik maas falls in the month when Sankranti (the first day of the month) does not fall within one month. When there are two Sankrantis within one month then it becomes the lunar year with one reduced month.
During Mal Maas people keep fasting. Hindus believe, Pursottam Mas is capable of washing away all sins bringing happiness and prosperity. During this month huge amount of charities are performed.
One of our religious story states, in ancient times, King Nahush took this fasting and hence he was released from all burdens and was given the throne of Indra.
According to Devi Bhagavat that charity good deeds and fasting carried out during this Mal Maas is sure to get good result.
Mal Maas or Adhik Maas is also called Purushottama Mahina. There is a story behind adoption of this name.
In the beginning there were only 12 months. All those months were assigned to and named after 12 gods. Astrologers when found Adhik Maas (extra month) and presented its importance, there was a problem for name and assignment. Then lord of the lords Vishnu agreed that the month should be assigned to him and he gave the name Purushottam Maas to this month.
Lord Vishnu also pointed that good deeds which could not be performed in any other months if done during this month can give equal results. Since then adhik maas is also known as the Purushottama Maas. Hence this month is known for doing good deeds which could not be done in regular months due to various reasons.
Purushottam Granth is the book explaining the significance and importance of this month. Pursottam mahatmey is recited in temples during this month. It is also believed that by reading and listening to pursottam mahatmaye purnya (dharma) can be achieved.
Puranas also praises about Adhik Maas. It is believed that recitation of Srimad Bhagavat Purana and Bhagavad Gita during this month gives best results. The Brata begins from the first day and ends on the last day of Adhik Maas.
Waking up early in the morning and worshipping Radha-Krisha or Lakshmi-Narayana and Sodashopachara puja has to be performed during the period of Brata. One time vegetarian meal is eaten in the evening.