Amanta vs Purnimanta: Why Hindu Month Names Differ Across India

In the lunisolar Hindu calendar a month follows the Moon's cycle from one phase to the next. The twelve names are Chaitra, Vaishakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha, Pausha, Magha and Phalguna. Each month has a bright fortnight (Shukla Paksha, waxing Moon) and a dark fortnight (Krishna Paksha, waning Moon).

The Amanta system ends the month at the new moon (Amavasya). It is used in Maharashtra, Gujarat, Karnataka, Andhra Pradesh, Telangana and Goa, among others. The Purnimanta system ends the month at the full moon (Purnima) and is used across most of North India, including Uttar Pradesh, Bihar, Rajasthan, Madhya Pradesh, Punjab and Haryana.

The bright fortnight has the same month name in both systems. The difference is only in the dark fortnight: in the Purnimanta system it already belongs to the next month, while in the Amanta system it still belongs to the month that started at the previous new moon.

Take the fortnight that follows the full moon of Ashadha. A North Indian panchang calls its tithis Shravana Krishna Paksha, since the month has "rolled over" at Purnima. A Gujarati or Marathi panchang calls the same fortnight Ashadha Krishna Paksha, because the month is not yet over. Same days, same Moon, two names.

This is why a festival like Janmashtami is described as Bhadrapada Krishna Ashtami in an Amanta calendar and Shravana Krishna Ashtami in a Purnimanta one, yet is celebrated on the same civil day.

A lunar year is about 11 days shorter than a solar year. To keep festivals in their seasons, an extra month (adhika masa) is inserted roughly every 32 to 33 months. It is added when a lunar month happens to contain no Sun sign change (sankranti). Very rarely, a month is dropped (kshaya masa). In an adhika year the same-named month appears twice, the first being the adhika month, which is generally not used for major ceremonies such as weddings.

Several regions use months defined by the Sun's entry into a zodiac sign rather than by the Moon. Tamil Nadu, Kerala, Bengal, Odisha, Assam and Nepal follow solar calendars, so their months run from one sankranti to the next and their new year falls around 14 April (or, for the Malayalam Kollavarsham, in August). Tithis are still used in these regions for religious observances, layered on top of the solar civil month.

Different regions also start the year on different days. Ugadi and Gudi Padwa mark the Chaitra Shukla Pratipada new year in the Deccan; Gujarat begins the Vikram Samvat year the day after Diwali, at Kartika Shukla Pratipada; solar-calendar regions celebrate at Mesha Sankranti. This is why one region can be in a different numbered year from another for part of the year.

If a festival date seems to disagree between two sources, check three things: which month system each uses, which city's sunrise it is calculated for, and whether the observance uses the tithi at sunrise or at another time of day. Usually the underlying astronomy agrees and only the naming or the rule differs.