How Hindu festival dates are actually calculated
Deepavali fell on 20 October in 2025 and falls on 8 November in 2026. Nothing was moved and nobody chose those dates: they are the same instruction, applied to a sky that does not divide neatly into years. Here is the machinery.
The month runs from new moon to new moon
A lunar month is the time from one new moon to the next — about 29.53 days. Twelve of them make roughly 354 days, eleven short of a solar year, which is why any festival tied to the Moon drifts about eleven days earlier each year against the Gregorian calendar.
Where a month starts is itself a regional choice. In the amānta reckoning of the south and west, the month begins at the new moon. In the pūrṇimānta reckoning of much of the north, it begins at the full moon. The same fortnight therefore carries different month names in the two systems — one reason a festival can be described as falling in Māgha by one source and Phālguna by another while both mean the same night.
How a month gets its name
Each lunar month takes the name belonging to the sign the Sun occupies at its new moon. A new moon with the Sun in Mīna opens Chaitra; the Sun in Meṣa opens Vaiśākha, and so on round the circle. That single rule ties the lunar calendar to the solar year and keeps the months roughly in their seasons.
The leap month
The tie is not exact, and the correction is elegant. The Sun moves into a new sign about every 30.4 days, while a lunar month lasts 29.53. Occasionally a whole lunar month passes with no solar ingress in it at all — and that month is adhika māsa, an intercalary month. It takes the name of the month that follows, and it carries no festivals: they belong to the true month of the same name that comes after.
This happens roughly every 32 or 33 months. In the range this site computes, leap months fall in 2026 (Jyeṣṭha), 2029 (Chaitra), 2031 (Bhādrapada) and 2034 (Āṣāḍha).
The tithi, and the moment that decides the day
Within a month, a festival is a tithi — one of thirty lunar days, each the time the Moon takes to gain 12° on the Sun. Ganesh Chaturthi is the fourth tithi of the waxing fortnight of Bhādrapada; Deepavali is the new moon that closes Āśvina.
But a tithi rarely lines up with a calendar day. It can begin at three in the afternoon and end at one the next day. So which date gets the festival? That is settled by which moment the observance belongs to, and it differs by festival:
| Reference moment | Festivals decided by it |
|---|---|
| Sunrise | Most of them — Rāma Navamī, Ugādi, Raksha Bandhan, the Ekādaśīs |
| Dusk (pradoṣa) | Deepavali, Dhanteras, Naraka Chaturdaśī, Chhath's evening arghya |
| Afternoon (aparāhṇa) | Vijayadaśamī, Bhai Dooj |
| Midnight (niśītha) | Mahā Śivarātri — and, in one of two traditions, Janmāṣṭamī |
Deepavali is the clearest case. The rule is that Amāvāsyā must be running at dusk, when the lamps are lit. In 2026 the new moon falls at midday on 9 November, so Amāvāsyā is already under way by the evening of the 8th — and that is the day the festival is kept, not the 9th when the conjunction actually completes.
Why two calendars differ by a day
Three honest reasons, none of them error:
- Sunrise is local. Whether a tithi is running "at sunrise" depends on where the sun rises. A calendar computed for Chennai and one computed for Delhi can put the same festival on different dates.
- Traditions genuinely differ. Janmāṣṭamī is kept a day apart by the smārta and vaiṣṇava reckonings in many years — both correct within their own rule.
- Kṣaya tithis. A short tithi can begin after one sunrise and end before the next, touching no reference moment at all. The almanac convention is to keep the festival on the day the tithi begins.
The festivals that are not tithis at all
- Solar: Makar Sankrānti and Pongal are the Sun's entry into Makara; Vishu and Puthāṇḍu its entry into Meṣa. These barely move — mid-January and mid-April every year — because they are solar, not lunar. Where the ingress falls after sunset, the southern convention moves the observance to the next day.
- Nakshatra: Onam is the Thiruvoṇam star in the Malayalam month of Chingam; Karthigai Deepam is Kṛttikā in the Tamil month of Kārthigai, read at dusk when the lamp is lit. A solar month can hold the same star twice — Chingam did in 2024 — and then the tithi it falls with decides which one is the festival.