Masik shivaratri and the nishitha rule

The chaturdashi covering nishitha — the middle of the night — takes the day. If two days qualify, the later one.

Nishitha is the middle of the night — the eighth part of the night by the usual reckoning, sitting either side of the midpoint between sunset and sunrise. It is not midnight on a clock.

The rule

The chaturdashi that covers nishitha takes the observance. Where the chaturdashi covers nishitha on two consecutive nights, the later night takes it.

That tie-break matters more than it sounds. A chaturdashi long enough to span two nishithas is not rare, and without the rule an almanac would have to choose arbitrarily.

Why it is not a sunrise rule

The observance is nocturnal, so the test is nocturnal. Applying a sunrise test to Shivaratri would put it on the wrong night roughly whenever the chaturdashi begins during the day — which is most of the time.

This is the same shape as pradosham, which is tested after sunset, and sankashti, which is tested at moonrise. The panchangam does not use one boundary for everything, and a site that behaves as though it does will print some dates wrongly.

The dates this gives

Applied to September and October 2026, the rule lands on 9 September and 8 October — the nights the established almanacs print.