Months of Hindu calendar are the distinctive feature that has undergone many changes in the process of regionalisation, and today there are several regional Indian calendars, as well as an Indian national calendar.

Hindu Calendar is inherited from a system first enunciated in Vedanga Jyotisha of Lagadha, a late BCE adjunct to the Vedas, standardized in the Surya Siddhanta (3rd century CE) and consequently reformed by astronomers such as Aryabhata (499 CE), Varahamihira (6th c. CE), and Bhaskara (12th c. CE). There are differences and regional variations that thrive in these computations, but the following is a general overview of Hindu lunisolar calendar. When a new moon occurs before sunrise on a day, that day is said to be the first day of the lunar month. So it is obvious that the end of the lunar month will correspond with a new moon. A lunar month of Hindu calendar has 29 or 30 days (according to the movement of the moon).

The tithi at sunrise of a day is the only indication 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 and in such a case, the latter is called an adhika tithi where adhika means "extra". Sometimes, one tithi may never reach 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".

The months of Hindu Calendar are known as the lunar months that are twelve in number. They are as follows -

Chaitra	Ashadha	Ashwin	Paush
Vaishakh	Shravana	Kartik	Magh
Jyaishtha	Bhadrapad	Margashirsha	Phalgun

Determining which name a month of Hindu calendar takes is somewhat indirect process. It is based on the rashi into which the sun moves within a lunar month, i.e. before the new moon ending the month. There are twelve rashi names, according to the months of Hindu calendar. When the sun moves into the Mesha r?shi in a lunar month, then the name of the lunar month is Chaitra. When the sun moves into Vrishabha, then the lunar month is Vaishakh. Thus the phenomena of naming the months of Hindu calendar continue.

The Sanskrit grammatical derivation of the months of Hindu Calendar, Chaitra and others is the (lunar) month which has its central full moon occurring at or near the nakshatra Chitr? is called Chaitra. Similarly, for the nakshatras Vishakha, Jyeshtha, (P?rva) Ash?dh?, Shravan, Bh?drapad, Ashvin? (old name Ashvayuj), Krittik?, Mrigash?rsha, Pushya, Megh? and (P?rva/Uttara) Phalgun? the names Vaish?kh and others are derived. The months of Hindu Calendar are split into two pakshas of 15 days. The waxing paksha is called shuklapaksha, light half, and the waning paksha the krishnapaksha, dark half.

There are two different systems for structuring the months of Hindu Calendar:

amanta or mukhya mana system - a month begins with a new moon, mostly followed in the southern states.

purnimanta or gauna mana system - a month begins with a full moon, followed more in the North states.

The extra months of Hindu calendar are the ones when the sun does not at all move into any r?shi but simply keeps moving within a rashi in a lunar month, then that lunar month will be named according to the first forthcoming transit. It will also take the epithet of adhik or "extra". For example, if a lunar month moves beyond without a solar transit and the next transit is into Mesha, then this month without transit is to be labeled adhik Chaitra. The next month will be labeled according to its transit as usual and will get the epithet nija ("original") or shuddha ("clean").

An adhik masa occurs once every two or three years (meaning, with a gap of one or two years without adhik masas Extra Month, or adhik mas masa (mas = lunar month) or purushottam mas). It is known so to give it a religious name, purushottam = Krishna and comes every 32.5 months. Thus 12 Hindu mas (m?sa) is equal to approximate 356 days, while solar year have 365 or 366 (in leap year) which create difference of 9 to 10 days, which is divided every 3rd year. But no extra months of Hindu Calendar falls during Kartik to Maha.

If the sun transits into two rashis within a lunar month, then the months of Hindu calendar will have to be labeled by both transits and will take the epithet kshay or "loss". They are considered to be a "loss" because in this case, there is only one month labeled by both transits. If the sun had moved into only one rashi in a lunar month as usual, there would have been two separate months labeled by the two movements in question. For example, if the sun moves into Mesh and Vrishabh in a lunar month, then it will be called Chaitra-Vaishaakh kshaya. There will be no separate months made Chaitra and Vaishakh. Akshay masa occurs very rarely. Known gaps between occurrences of kshaya masas are 19 and 141 years. The last lost month of Hindu Calendar was in 1983. January 15 through February 12 was Pausha-M?gha kshay. February 13 onwards was (adhik) Ph?lguna.

Among normal months, adhika months, and kshaya months, the earlier are considered "better" for religious purposes. That means, if a festival will fall on the 10th tithi of the ?shvayuja month (this is called Vijayadasham?) and there are two ?shvayuja months caused by the existence of an adhika ?shvayuja, the first adhika month will not see the festival, and the festival will be observed only in the second nija month of Hindu Calendar. However, if the second month is ?shvayuja kshaya then the festival will be observed in the first adhika month itself.

When two months of Hindu calendar are rolled into one in the case of a kshaya m?sa, the festivals of both months will also be rolled into this kshaya m?sa. For example, the festival of Mah?shivar?tri which is to be celebrated on the fourteenth tithi of the M?gha krishna paksha was, in 1983, observed on the related tithi of Pausha-M?gha kshaya krishna paksha, since in that year, Pausha and M?gha were rolled into one, as mentioned above. When two months of Hindu calendar are rolled into one in the case of a kshaya m?sa, the festivals of both months will also be rolled into this kshaya m?sa.