Adhik Maas or Purushottama Masa is the extra month added to a Hindu lunar calendar once in 32 months to make the lunar calendar similar to the solar calendar. There is an interesting story that associates Adhik Maas or Purushottama Masa with Hindu God Vishnu.
The 12 months in a traditional calendar is assigned to particular Hindu God. But when the 13 month in a lunar calendar was created by Rishis and Saints, it was not assigned to any God. Thus it acquired the name of Mal Maas or Malimmucha. This was like a stigma to the extra month.
Now the Adhik Maas felt disillusioned and sad and approached Lord Vishnu. Adhik Masa complained that as he was not assigned to any God he will be rejected by humans. So the Adhik Masa sought the help of Lord Vishnu to escape from this tricky situation.
Lord Vishnu took pity on Adhik Masa and assigned it to him especially to Lord Krishna. He also gave the name Purushottama Maas to the month.
Lord Vishnu also said that the merit acquired in this month through good deeds, japa, charity, austerities etc is equal to such acts performed in all the 12 months of a year. Since then it is known as the Purushottama Maas and acquired greater significance than the other months.