When a new moon occurs before sunrise on a day, which that day is said to be the first day of the lunar month. So, it is evident that the end of the lunar month will coincide with a new moon. A lunar month has twenty nine or thirty days, which is according to the movement of the moon.



The tithi at sunrise of a day is the only label of the day. There is no running day number from the first day to the last day of the month. Sometimes two successive days have the same tithi. In such a case, the latter is called an adhika tithi where adhika means extra. Sometimes, one tithi may never touch a sunrise, and hence no day will be labeled by that tithi. It is then said to be a tithi kshaya where kshaya means loss.

Determining which name a lunar month takes is some what indirect. It is based on the rashi into which the sun transits within a lunar month. For instance, before the new moon ending the month. There are twelve rashi names and there are twelve lunar month names. The Sanskrit grammatical derivation of the lunar month names Chaitra is the lunar month which has its central full moon occurring at or near the nakshatra Chitra is called Chaitra. Similarly, for the nakshatras Vishakha, Jyeshtha, Ashadha, Shravan, Bhadrapad, Ashvini, Krittika, Mrigashirsha, Pushya, Megha and Phalguni, which is where the names are derived.

The lunar months are split into two pakshas of fifteen days. The waxing paksha is called shuklapaksha, light half, and the waning paksha the krishnapaksha, dark half. There are two different systems for making the lunar calendar, which are known as the amanta and the purnimanta. The amanta or mukhya mana system is when a month begins with a new moon, mostly followed in the southern states. The purnimanta or gauna mana system is when a month begins with a full moon, followed more in the North.

When the sun does not at all transit into any rashi, but simply keeps moving within a rashi in a lunar month, then that lunar month will be named according to the first upcoming transit. It will also take the epithet of adhik or extra. For example, if a lunar month elapsed without a solar transit and the next transit is into Mesha, then this month without transit is labeled adhik Chaitra. The next month will be labeled according to its transit as usual and will get the epithet nija or shuddha.

An adhik masa occurs once every two or three years, which means with a gap of one or two years without adhik masas. Extra Month, or adhik mas masa, with mas meaning lunar month, or purushottam mas falls every thirty two and a half months. It is known so to give it a religious name, purushottam krishna. Thus the twelve Hindu mas are equal to approximate three hundred fifty six days, while solar year has three hundred sixty five or three hundred sixty six in leap year, which create difference of nine to ten days, which is subset every third year, but no adhik mas falls during Kartik to Maha.