Sankranti and the calendar date

A sankranti marks the Sun’s entry into a zodiac sign. Regional calendar rules determine which civil day marks the start of the solar month.

The astronomical transition occurs at one instant. A calendar then applies its convention to assign that transition to a date.

The Tamil rule

If the sankranti occurs before local sunset, the same civil day is taken as the first day of the new solar month. Otherwise, the new month begins on the following day. Tamil calendars apply this rule to the Vakya sankranti, which can fall a few hours later than the Drik one. The Tamil pages of this site take each month's first day from published Tamil calendar listings.

The Malayalam rule used here

The cutoff is the beginning of aparahna, three fifths of the way from sunrise to sunset. A sankranti before that point starts the month on the same day; a later one starts it on the following day.

Why two calendars may differ

A transition between the two cutoffs can produce different month-start dates under these rules: in 2026 Purattasi began on 17 September by the Drik sankranti and on 18 September in the Tamil calendars.

A festival may also have its own observance rule. Its date should not be inferred from the month-start convention alone.