The calendar “date” that we are so familiar with in our daily life is based on the solar calendar. The English calendar is a solar calendar.  The basis for the solar calendar is the rotation of the earth around the Sun.  It takes the earth approximately 365 ¼ days to complete its rotation around the Sun. The English calendar that most of us use today divides the 365 days of earth’s period of rotation around the Sun in twelve months.  The leap year, which occurs once every four years, accounts for ¼ day per year.

Similar to the solar calendar a lunar calendar is also popular and widely used in the Asian countries such as China, Pacific-rim countries, Middle East countries, and India.  The lunar calendar, which is believed to have originated in India, has been around for a very long time, even long before the solar calendar. 

The lunar calendar is based on the moon’s rotation around the earth.  The lunar month corresponds to the one complete rotation of the moon around the earth.  Since this period of rotation of the moon around the earth varies, the duration of the lunar month also varies.  On average, the lunar month has about 29 ½ days.  In addition to the moon’s rotation around the earth, the lunar year is based on the earth’s rotation around the Sun.  In general, the lunar year has twelve lunar months of approximately 354 days, thus making it shorter by about 11 days than the solar year.  However, the lunar calendar accounts for this difference by adding an extra lunar month about once every 2½ years.  This extra lunar month is known as the “Adhik Maas” in India (Adhik means extra and the Maas means month). The concept of placing of the extra month (meaning why and when should it be inserted between certain lunar months) is as follow. 

According to sidereal zodiac system the Sun enters the first zodiac sign Aries (Mesh) on about April 15 of every year. And about 15th of every month the Sun enters the next sidereal zodiac sign. For example, as we know, every year on the day of Makar Sankranti 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 ½ 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 ½ 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 the Adhik Maas (the extra month) is similar to the “Blue Moon” in the West, which occurs almost with the same frequency of 2 ½ years. The 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,30th,or 31st of the month). 

Recall that the entry of the Sun in a sidereal zodiac sign occurs around the middle of the calendar (solar) month (near 15th of a month), thus, the Sun stays in a sidereal zodiac sign from about 15th of a month to about 15th of the next month.  Since for the Adhik Maas to occur, two new moons must occur during when the Sun remains in the same zodiac sign. Consequently, those new moons must occur near the15th of the successive months.  As a result, around the month of the Adhik Maas, the successive full moons very likely occur about two weeks prior and during the Adhik Maas or during and two weeks after the Adhik Maas . Indeed, the occurrence of the blue moon usually takes place either during or two weeks after the Adhik Maas .