Months of the lunisolar calendar[edit]

The astornomical basis of the Hindu lunar day. Also illustrates Kshaya Tithi (Vaishaka-Krishna-Chaturdashi (i.e. 14th)) and Adhika Tithi (Jyeshta- Shukla-Dashami(i.e. 10th))

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 evident that the end of the lunar month will coincide with a new moon. A lunar month has 29 or 30 days (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. This has some unique results, as explained below:
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 Kṣaya where Kṣaya means "loss".

Month names[edit]

There are 12 months in Hindu lunar Calendar:

  1. Chaitra (चैत्र, చైత్రం) Meṣa (Aries)[5]
  2. Vaiśākha (वैशाख, వైశాఖం) Vṛṣabha (Taurus)[6]
  3. Jyaiṣṭha (ज्येष्ठ, జ్యేష్ఠ) Mithuna (Gemini)[7]
  4. Āṣāḍha (आषाढ, ఆషాఢం) Karka (Cancer)[7]
  5. Śrāvaṇa (श्रावण, శ్రావణం) Siṃha (Leo)[7]
  6. Bhādrapada or Bhādra also Proṣṭhapada (भाद्रपद,भाद्र,प्रोष्ठपद, బాద్రపదం) Kanyā (Virgo)[7]
  7. Āśvina in,sometimes Aśvayuja ( आश्विन,अश्वयुज, ఆశ్విజం) Tula (Libra)[7]
  8. Kārtika (कार्तिक, కార్తీకం) Vṛścika (Scorpio)[7]
  9. Agrahāyaṇa or, Mārgaśīrṣa (मार्गशीर्ष,अग्रहायण, మార్ఘశిరం) Dhanus (Sagittarius)[7]
  10. Pauṣa (पौष, పుష్యం) Makara (Capricorn)[7]
  11. Māgha (माघ, మాఘం) Kumbha (Aquarius)[7]
  12. Phālguna (फाल्गुन, ఫాల్గుణం)Mīna (Pisces)[7]

Determining which name a lunar month takes is somewhat indirect. It is based on the rāshi (Zodiac sign) into which the sun transits within a lunar month, i.e. before the new moon ending the month.
There are 12 rāśi names, there are twelve lunar month names. When the sun transits into the Meṣa rāśi in a lunar month, then the name of the lunar month is Caitra. When the sun transits into Vṛṣabha, then the lunar month isVaiśākha. So on.

If the transits of the Sun through various constellations of the zodiac (Rāśi) are used, then we get Solar months, which do not shift with reference to the Gregorian calendar. The Solar months along with the corresponding Hindu seasons and Gregorian months are:
The Sanskrit grammatical derivation of the lunar month names Caitra etc., is: the (lunar) month which has its central full moon occurring at or near the Citrā nakṣatra is called Caitra. Another example is let's say when Pūrṇimā occurs in or near Viśākha nakṣatra, this in turn results to the initiation of the lunar month titled Vaiśākha Māsa.[8]
Similarly, for the nakṣatra-s Viśākha, Jyeṣṭhā, (Pūrva) Āṣāḍhā, Śravaṇa, Bhādrapadā, Aśvinī (old name Aśvayuj), Kṛttikā, Mṛgaśiras, Puṣya, Meghā and (Pūrva/Uttara) Phalguṇī the names Vaiśākha etc. at pūrṇimā, the other Lunar names are derived subsequently.

Extra months (Adhika Māsa)[edit]

The astronomical basis of the Hindu lunar months. Also illustrates Adhika Masa (Year 2-Bhadrapada) repeats twice; the first time the Sun moves entirely within Simha Rashi thus rendering it an Ashika Masa

When the sun does not at all transit into any rāśi but simply keeps moving within a rāśi in a lunar month (i.e. before a new moon), then that lunar month will be named according to the first upcoming transit. It will also take the epithet of adhika or "extra". For example, if a lunar month elapsed without a solar transit and the next transit is into Meṣa, then this month without transit is labeled Adhika Caitra Māsa. The next month will be labeled according to its transit as usual and will get the epithet nija ("original") or Śuddha ("unmixed"). In the animation above, Year 2 illustrates this concept with Bhadrapada repeating twice; the first time the Sun stays entirely within Simha rashi thus resulting in an Adhika Bhakradapada.
Extra Month, or adhika māsa (māsa = lunar month in this context) falls every 32.5 months. It is also known as puruśottama māsa, so as to give it a devotional name. Thus 12 Hindu mas (māsa) 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 offset every 3rd year. No adhika māsa falls during Kārtika to Māgh.
A month long fair is celebrated in Machhegaun during adhika māsa. It is general belief that one can wash away all one's sins by taking a bath in the Machhenarayan's pond.

Lost months (Kṣaya Māsa)[edit]

If the sun transits into two rāshis within a lunar month, then the month will have to be labeled by both transits and will take the epithet kṣaya or "loss". There is considered to be a "loss" because in this case, there is only one month labeled by both transits. If the sun had transited into only one raashi in a lunar month as is usual, there would have been two separate months labeled by the two transits in question.
For example, if the sun transits into Meṣa and Vṛṣabha in a lunar month, then it will be called Caitra-Vaiśākha kṣaya-māsa. There will be no separate months labeled Caitra and Vaiśākha.
A Kṣaya-Māsa occurs very rarely. Known gaps between occurrence of Kṣaya-Māsas are 19 and 141 years. The last was in 1983. January 15 through February 12 were Pauṣa-Māgha kṣaya-māsa. February 13 onwards was (Adhika) Phālguna.

Special Case:
If there is no solar transit in one lunar month but there are two transits in the next lunar month,

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  • the first month will be labelled by the first transit of the second month and take the epithet Adhika and
  • the next month will be labelled by both its transits as is usual for a Kṣaya-Māsa

This is a very very rare occurrence. The last was in 1315. October 8 to November 5 were Kārtika Adhika-Māsa. November 6 to December 5 were Kārtika-Mārgaśīrṣa Kṣaya-Māsa. December 6 onwards was Pauṣa.

Religious observances in case of extra and lost months[edit]

Among normal months, adhika months, and kshaya months, the earlier are considered "better" for religious purposes. That means, if a festival should fall on the 10th tithi of the Āshvayuja month (this is called Vijayadashamī) and there are two Āśvayuja (Āśvina)' months caused by the existence of an adhika Āśvayuja, the first adhika month will not see the festival, and the festival will be observed only in the second nija month. However, if the second month is āshvayuja kshaya then the festival will be observed in the first adhika month itself.
When two months are rolled into one in the case of a kshaya māsa, the festivals of both months will also be rolled into this Kṣaya Māsa'. For example, the festival of Mahāshivarātri which is to be observed on the fourteenth tithi of the Māgha Kṛṣṇa-Pakṣa was, in 1983, observed on the corresponding tithi of Pauṣa-Māgha Kṣaya Kṛṣṇa-Pakṣa, since in that year, Pauṣa and Māgha were rolled into one, as mentioned above. When two months are rolled into one in the case of a Kṣaya Māsa, the festivals of both months will also be rolled into this kṣaya māsa.