There are two traditions being followed with respect to the start of the month. Amavasyant (Amanta) tradition followed mainly in the western and southern states of India (namely Andhra Pradesh, Goa, Gujarat, Karnataka, Maharashtra, and Tamil Nadu) considers a new moon occurring before sunrise on a day to be the first day of the lunar month. Purnimant tradition, on the other hand, considers the next day of a full moon to be the first day of the lunar month. This tradition is chiefly followed in the northern and eastern states of India (Bihar, Himachal Pradesh, Madhya Pradesh, Punjab, Odisha, Rajasthan, and Uttar Pradesh). Having the two active traditions in practice would also mean that while the month names of the Hindu lunar calendar remains the same, there is on an average 15 days’ difference in starting and ending of the month between the two traditions. This has its effects in the dates of recurring annual events such as the holy month of Śrāvaṇa. For example, between the followers of the two traditions, the start of Śrāvaṇa month and its religious abstinence and observations will be deferred by 15 days for the followers of Amavasyant tradition.
A month contains two halves (pakṣas), the waxing half and the waning half. Each half is 15 phases long.
There are 12 months in Hindu lunar calendar (Sanskrit: मासाः):
- Chaitra
- Vaiśākha
- Jyeṣṭha
- Āṣāḍha
- Śrāvaṇa
- Bhādrapada, Bhādra or Proṣṭhapada
- Ashvin or Ashvija
- Kārtika
- Agrahāyaṇa, Mārgaśīrṣa
- Pauṣa or Pushya
- Māgha
- Phālguna
Names of months are interesting. It depends on the asterism name, the moon is in at full moon day of the month. for e.g. Chaitra month is when full moon is in Citrā nakṣatra , Ashvin month is when full moon is in Aśvinī nakṣatra
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 isChaitra which has both Mīna rāśi and Meṣa rāśi . When the sun transits into Vṛṣabha rāśi, then the lunar month is Vaiśākha which has both Meṣa rāśi and Vṛṣabha rāśi. So on.
Purshottam maas or adhika maas is an extra month or thirteen in the Hindu calendar. This is been done for bridging of the lunar and solar calendars