One sankranti, five rules

The Sun enters a sign at one instant everywhere. Which calendar day that instant belongs to is decided regionally, and the regions disagree.

A sankranti is an astronomical event: the Sun crosses into the next sign at a definite moment, the same moment for everyone. Solar new year festivals — Pongal, Vishu, Mesha Sankranti — hang off that moment.

But a festival needs a day, and turning an instant into a day is a convention, not an observation. There are several, and they are regional.

The rules

• Tamil. Sankranti before sunset — the same day. Otherwise the next.
• Kerala, the Malabar Rule. Before aparahna — three fifths of the daylight — the same day. Otherwise the next.
• Odisha. Before midnight, the same day.
• Bengal. Between sunrise and midnight, the next day. After midnight, the third day.
• Orissa, by Sewell's note. The same day, always.

Take one sankranti at, say, four in the afternoon and these give at least three different civil dates. None of them is a mistake.

Vishu

Vishu is the first sunrise after the Mesha sankranti, which is a sunrise rule rather than one of the above. The Calendar Reform Committee lists Vishu under Meshadi with a midnight-based civil day for transit festivals, and Vaishakhi as sunrise-based — two different treatments in one paragraph of one report, which is a fair picture of the territory.

Reading it against a printed almanac

If a solar festival looks a day out against the almanac you keep, the day-rule is the first thing to check. An almanac published for Kerala and one published for Tamil Nadu apply different tests to the same instant, so they will part company whenever a sankranti falls in the afternoon.