For example, as we know, every year on MakarSankranti the Sun enters the sidereal zodiac sign Capricorn on about January 14. While Sun remains in a zodiac sign for approximately one month, the Moon travels through all twelve zodiac signs in about 27 days. As a result, on average, once about every two and half 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, the month during which two new moons occur, happens once about every 2 years. The lunar month corresponding to the period between these two new moons is treated as the extra month or the Adhik Mas.
Blue moon is the second full moon when two full moons occur in the same month.
Since for Adhik Mas to occur, two new moons must occur during when sun remains in the same zodiac sign. Consequently, those new moons must occur near 15 th of the successive months. As a result, around the time of Adhik Mas, the successive full moons are very likely to occur near the beginning and the end of the same month. Indeed, the occurrence of the blue moon usually precedes the Adhik Mas. The Indian lunar year begins on the new moon day, which usually occurs near the beginning of the Spring season.