Amanta and purnimanta

A lunar month is one full round of the moon's shapes. It takes about twenty-nine and a half days. A round has no natural starting point, so somebody has to choose where to cut it, and the moon gives two obvious places. One is the full moon. The other is the no-moon night, when nothing is showing.

North India cuts at the full moon. That system is called purnimanta, which means "ending at the full moon". Gujarat, Maharashtra and the states down the peninsula cut at the no-moon night. That system is called amanta, "ending at the new moon".

Every month has a bright half, when the moon is filling, and a dark half, when it is emptying. Because the two systems cut at opposite points, they take those halves in opposite order. This has one visible effect. During the bright half, both systems use the same month name. During the dark half, they differ by exactly one name, and the northern name is one month ahead.

So full moons are called the same thing everywhere. No-moon days are not. And any festival in a dark fortnight, such as Janmashtami, gets two month names and one date.

The rough north-and-south rule has exceptions. Gujarat is amanta and is not southern. Tamil Nadu, Kerala, Bengal, Assam and Odisha run their everyday calendars on solar months instead, so they are answering a different question. And the boundary has moved before: dated inscriptions show the northern system in use in parts of the south until about the ninth century.