The Hindu Calendar is inherited from a system first enunciated in the 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 century CE) and Bhaskara (12th century CE). There are differences and regional variations that thrive in these computations.
Months of Hindu Calendar
The Hindu calendar marks 12 months by the same cycle as the lunar phases, approximately 29.5 days each month depending on celestial movement. The month is broken down into 2 fortnights, a waning moon and a waxing moon, each lasting 15 lunar days. In some months, a day of the cycle may need to be dropped to correlate with a shorter lunar cycle. The first day of the month varies from calendar to calendar. Generally, in North India, the full moon marks the first day of the month, while in South India, the occasion is marked by the new moon.
Listed below are the names of the 12 months of Hindu Calendar:
• Chaitra (March to April)
• Vaisakha (April to May)
• Jyestha (May to June)
• Asadha (June to July)
• Shraavana (July to August)
• Bhadra (August to September)
• Ashvina (September to October)
• Kartika (October to November)
• Agrahayana (November to December)
• Pausha (December to January)
• Magha (January to February)
• Phalguna (February to March)
Naming of a Month of Hindu Calendar
Determining which name a month of Hindu calendar takes is somewhat indirect process. It is based on the Rashi or the zodiac signs into which the sun moves within a lunar month, i.e. before the new moon ending the month. There are 12 Rashi names, according to the months of Hindu calendar. When the sun moves into the Mesha rashi 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.
According to the Atharva Veda, the months of Hindu Calendar are derived from the name of the Nakshatras. Chaitra and others is the (lunar) month which has its central full moon occurring at or near the Nakshatra Chitra and thus is called Chaitra. Similarly, for the nakshatras Vishakha, Jyeshtha, Ashadha, Shravan, Bhadrapad, Ashvina, Krittika, Mrigashira, Pushya, Megha and Phalguna, the names Vaishakh and others are derived. The months of Hindu Calendar are split into 2 pakshas of 15 days. The waxing Paksha is called ‘Shuklapaksha’, light half, and the waning Paksha the ‘Krishnapaksha’, dark half.
Structure of the Months of Hindu Calendar
There are 2 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 dates of many, but not all, Hindu festivals are determined according to the months of Hindu calendar. In most cases, the festivals coincide with the Full Moon or the New Moon, or they are celebrated on the day after the Moon phase. Festivals based on the Hindu calendar include Maha Shivaratri, Holi, Guru Purnima, Ganesh Chaturthi and Diwali.
Extra Months of Hindu Calendar
The extra months or Adhika Masa of Hindu calendar are the ones when the sun does not at all move 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 forthcoming transit. It will also take the epithet of Adhika or "extra" which happens once every 2 or 3 years.