Source : Complied from various published articles and online resources on Adhik Maas
Adhik Maas (अधिक मास) means "additional month" or an "Extra Month." The lunar calendar adds one extra month every third year. Adhik maas is also known as "Mal Maas," "Purushottam Maas," or "Malimmacha." Adhik maas is a geophysical phenomenon behind which there is an accurate calculation of lunar months and the solar year. A Hindu Lunar month is 29.5 days long and a Solar month is 30 or 31 days long. The Hindu Lunar year consists of 354 days and Solar year consists of 365 days. So there is a difference of 11 days. This difference increases to 29 or 30 days after every 2 or 3 years. Hence an extra month is added to make the two calendars similar.
According to another belief, when there is no surya sankranti on any month that month is called as "Adhik Maas" or "Purushottam Maas" or "Mal Maas."
Adhik Maas Story
There are many stories behind the naming of this month as the Mal Maas and Purushottam Maas and many more. One of the most prominent story is that, this month did not belonged any lord and the other months refused to consider this month as a legitimate one and dubbed it as an abominable on like the stool and hence called it the Mal Maas. But, later it was raised to the class higher than that of the other months when Lord Vishnu or Purushottam gave his name assigned himself to this month, and henceforth it is known as Purushottam Maas.
Another version of this story is that, since Sun does not enter in any Rashis during this month and no auspicious rituals are performed , so people consider it as waste month. Getting this blot , the Adhik maas was grieved and went to Shree Hari Vishnu in Vaikunth taking the form of a human and started very hard and sincere prayers. Shri Hari Vishnu blessed him and took him to Golok which is difficult even for Yogies. Where Purushottam Bhagwan Krishna blessed him with boon and assured Vishnuji for the uplifting of Malmaas by giving his name Purushottam and equal status. Hence forth this month is known as Purushottam maas.
It is also believed that, Lord Vishnu also said that acquisition of merits during other months through good deeds, japa, austerities etc can be acquired by japa, austerities etc. carried out within this one month.
Another story relates to how, in ancient times, through the observance of Mal Maas Vrat, king Nahush was released from all bondage and acquired the throne of Indra.
Accoding to another legend, once Lakshmi ma asked Srihari Vishnu bhagwan, how to perform the Adhik Maas pooja and what are the rituals performed during this month. Then Srihari told her that he himself is the God for the Adhik maas with the name Purushottama. Punya kaarya lika snaana, japa, homaa will bring akshaya phala. He also told Lakshmi ma that those who do Not do any punya karya during this month will be getting daaridrya, putrashoka, etc Those who could not do for the whole month can do it atleast on Krishna paksha ashtami, navami, chaturdashi, Dwadashi, poornima etc.
In Vaishnav temples purushottam maas is celebrated very joyfully for the whole month. In the religious city ujjain purushottam maas is celebrated very great enthuiasm. It is believed that worshipping and performing religious duties in this month with pious and devoted mind will bring positive results.
The Importace of Adhik Month Mas lies in the fact that this is the time when one gets the chance to travel to the realm of self-revaluation, self-development, self-reflection, self-assessment, self-retrospection and introspection. Observing the rituals of this month thoroughly will enable a person to gain peace of mind. Many people read or listen to the book named, "Purushottama Granth" during Purushottama Maas (Adhik Maas). It is believed that by reading and listening this book during Adhik maas, high merits are acquired. The merit of Vrat in this month is greater than hundreds of Yagna. Regarding Adhik Maas (Mal Maas) Hemadri says that Adhik Maaas Vrat is capable of washing away all the sins of people who observe the Vrat. Those who perform Pooja, Vrat, Daan or other penance in this month will attain good health, wealth, and even Moksha from cycle of rebirth
Performing penances knowingly or unknowingly in this adhik masa in any form imbuse spiritual merit and attains peace Any graha dosh or specific dosh nivaran puja performed in purushottam masa to rectify the horoscope gives ten times more better result to the individual.