Why Diwali's date moves every year

A festival set by the Moon

The Gregorian calendar follows the Sun. Diwali follows the Moon. It is kept on Amāvāsyā, the new Moon, that closes the lunar month of Ashwina in the amanta reckoning, where months end at new Moon. In the purnimanta reckoning, where months end at full Moon, the same night falls in the dark half of Kartika. Both reckonings name one night; only the month label differs.

A lunar year of twelve months is about 354 days long, some eleven days shorter than the solar year. So Diwali usually comes about eleven days earlier than the year before, until the calendar inserts a thirteenth month, an Adhika māsa, and the date jumps about nineteen days later. Our engine's dates for New Delhi show the pattern: 31 October 2024, 20 October 2025, 8 November 2026 (after the adhika Jyeshtha of that summer), 29 October 2027 and 17 October 2028. The festival stays in autumn because of the leap month; it moves within autumn because of the Moon.

Amavasya is an angle, not a day

A Tithi, or lunar day, is the time the Moon takes to gain twelve degrees on the Sun. Amavasya is the thirtieth tithi, the last twelve degrees before the two bodies meet. It begins and ends at whatever clock time the geometry dictates and can last anywhere from about twenty to twenty-seven hours. It therefore rarely lines up with a civil day.

For the majority of observances the civil day takes its name from the tithi current at local sunrise. Diwali uses a different test, and that is where much of the confusion comes from.

The pradosh test

The Lakshmi Puja that defines Diwali is performed in the evening, in pradosha kala, the period after sunset. The word Pradoṣa names both this evening twilight and the trayodashi vrata kept in it. Almanacs commonly take pradosh as the first three of the fifteen Muhūrtas of the night, which in November comes to about two and a half hours, though definitions differ†.

Dharmashastra digests such as Kamalakara Bhatta's Nirnaya Sindhu (1612) and Kashinatha Upadhyaya's Dharma Sindhu (late 18th century) require that Amavasya be present during pradosh. So the question each year is: on which evening is Amavasya running after sunset? The rules generally applied are these:

• If Amavasya covers pradosh on only one evening, that day is Diwali.
• If it touches pradosh on two evenings, the second is usually taken.
• If it fully covers neither, the day on which it covers more of the evening is chosen.

The tie-breaks differ in detail between digests and regional traditions†.

A worked case: 2026

In New Delhi on 8 November 2026, the tithi at sunrise is still Chaturdashi, the fourteenth. Amavasya begins late in the morning and runs through the evening and the night, ending shortly after noon the next day, before sunset. By the sunrise rule the day would be named Chaturdashi; by the pradosh rule it is Diwali. A calendar that applied the sunrise rule mechanically would print the wrong evening.

Why almanacs sometimes split

Pradosh begins at sunset, and sunset differs by place. In 2024 the new Moon began in the afternoon of 31 October and ended in the early evening of 1 November. Every Indian city had Amavasya through the whole of pradosh on the 31st but only part of it on the 1st, and how much depended on how late the Sun set. Almanacs that read the rule as "the second evening, if Amavasya touches its pradosh" printed 1 November; those that asked for fuller coverage printed 31 October. Both groups were applying the same texts to the same sky. Our research note on five almanacs describes how to trace such differences.

The Lakshmi Puja window

Within pradosh, many almanacs print a narrower window: the time when a sthira (fixed) Lagna, or rising sign, is on the eastern horizon. In October and November the fixed sign Vrishabha rises in the early evening, so the printed puja time is the overlap of three things: Amavasya, pradosh and Vrishabha lagna. All three depend on longitude and latitude, so the window differs between Mumbai, Kolkata and Delhi on the same date. Traditions that keep a night worship, such as Kali Puja in Bengal, look instead to nishita, the muhurta around local midnight.

Five days, five tests

Diwali sits within a group of observances, each fixed by its own tithi and its own part of the day:

• Dhanteras on Trayodashi, the thirteenth, judged at pradosh.
• Naraka Chaturdashi on the fourteenth, judged before sunrise.
• Diwali on Amavasya, judged at pradosh.
• Govardhan Puja on the first tithi of the bright half of Kartika. In Gujarat the same day opens the new year of the Vikram Saṃvat, the era used in that region's calendar.
• Bhai Dooj on the second tithi, judged in the afternoon.

Because each test looks at a different part of the day, the "five days" occasionally become four or six.

What the date does not say

A festival date is the result of an astronomical computation and a rule of observance. Neither carries any promise about the year ahead. We describe the rules so that readers can see why a date was chosen and why another almanac may differ.

Try it

The Diwali page gives this year's date for your city with the Amavasya span, pradosh and the Vrishabha lagna window, each with the rule that produced it. The Festivals list shows the other four days fixed by the same reasoning, and the Panchang shows the tithi for any date.

† Marked dates or readings are debated among scholars or still being confirmed by our faculty.