The Solar year has 365 days whereas the Lunar has fewer days, because each month is roughly 29.53 days only according to Moon's speed. To bring the two years in sync, the Adhik Maas (an extra month) is added to the Lunar year once in approx 2 and 1/2 years. This is the month when the Moon is not near any Nakshatra (stellar constellation) and the Sun has not entered any Raashi or Sign of Zodiac. Complicated as this is, it is easy to remember that the Adhik Maas balances the Lunar and Solar calendars. Also because the earth's rotation around the Sun is slightly slower during the months of July-October, the Adhik Maas comes in this period only. Shraavan, Bhaadrapad, Aashwin are the common Adhik Maas months. In the year 2004, the Adhik Maas (month) started on July 18 and ended on August, 15.

This month is considered very auspicious and is especially dear to Lord Krishn’s heart. The following story, connected with Adhik Maas, is popularly known -

Hearing this, "Mal Maas" felt very sad and took his plight to Lord Krishn. He cried - “Why only me to carry the burden of all the sins?” Lord Krishn was filled with compassion. He said - “I bless you, O Mal Maas. Henceforth you will be named after me. You will be known as the Month of Purushottam (Purush means 'man', and Uttam means 'the best' - thus Puroshottam refers to the Highest Being, means 'God or Paramaatmaa), so you will be known as "Purushottam Maas" from today."

The Lord further stated: “Whosoever shall pray, keep fast, chant, sing, worship during the month of Purushottam, will be specially blessed, and get maximum recognition by me." Adhik Maas got very happy to hear this and never got sad after that.

Adhik Maas or Blue Moon

Now when this extra month is inserted between certain lunar months is as follow -

According to sidereal zodiac system Sun enters the first zodiac sign Aries (Mesh) on about April 15 of every year. And as the Sun takes 1 month (30 days) to move to another Sign, he moves in the next Sign about 15th of every month. For example, on Makar Sankranti, about January 14, the Sun enters the sidereal zodiac Sign Capricorn. While Sun remains in a zodiac Sign for approximately 30 days, the Moon travels through the zodiac Sign in about 27.5 days. As a result, on average, once about every 2 and 1/2 years, the entry of the Moon in the same zodiac Sign occurs twice while the Sun remains in the same sign.

In other words, when the Sun is traveling through the same zodiac Sign, in whichever month during which two new moons (Amaavasyaa) occur, happens once about every 2 and 1/2 years. The lunar month corresponding to the period between these two new moons is treated as the extra month or the Adhik Maas. Thus, if the Adhik Maas occurs at the beginning of the lunar month Chaitra, then it’s called as Adhik Chaitra, and the following lunar month would be then the regular or Neej lunar month Chaitra.

The concept of this Adhik Maas (the extra month) is similar to the “Blue Moon” in the West, which occurs almost with the same frequency of 2 and 1/2 years. Blue Moon is the second Full Moon when two Full Moons occur in the same month. Naturally the Blue Moon must occur towards the end of month (some where between 29th, or 30th, or 31st of the month).

Normally the entry of Sun in a sidereal zodiac Sign occurs around the middle of the calendar (solar) month (near 15th of the month), thus, the Sun stays in a sidereal zodiac Sign from about 15th of one month to about 15th of the next month. Since for Adhik Maas to occur, two New Moons must occur during when sun remains in the same zodiac Sign; consequently, those New Moons must occur near 15th of the successive months. As a result, around the time of Adhik Maas, the successive Full Moons are very likely to occur near the beginning and the end of the same month. Thus the occurrence of the Blue Moon usually precedes the Adhik Mas.