Diwali falls somewhere between mid-October and mid-November; Holi anywhere from late February to late March. The reason is that almost every Hindu festival is fixed to a tithi in a lunar month, and neither of those lines up with the Gregorian calendar.
Lunar months drift against the solar year
Twelve lunar months come to about 354 days, eleven short of the solar year. Left alone, every festival would slide eleven days earlier each year and eventually lap the seasons. The Hindu calendar prevents this by inserting an extra month — the adhika masa — roughly once every two and a half to three years, which pulls the festivals back by about a month.
The rule for when the extra month occurs is astronomical, not a fixed cycle. A lunar month is named by the sign the Sun enters during it. A month in which the Sun enters no new sign at all (no sankranti) has no name of its own, and becomes the adhika month; festivals are held in the regular month that follows. This site finds every new moon and every sankranti for the year and applies that rule directly.
Which day? It depends on the festival
A tithi lasts about a day but starts at any hour, so it usually spans two civil dates. Which one carries the festival is decided by a rule about when in the day the tithi must be current. The rules come from the nirnaya (decision) literature of the Dharmashastra, and they differ by observance:
| Rule | The tithi must be current… | Used for |
|---|---|---|
| Sunrise | at sunrise | Most vratas, Navratri, Chhath |
| Madhyahna | at midday | Rama Navami, Ganesh Chaturthi, Akshaya Tritiya |
| Aparahna | in the fourth fifth of daylight | Vijayadashami, Bhai Dooj |
| Pradosh | just after sunset | Diwali, Dhanteras, Tulsi Vivah |
| Nishita | at local midnight | Maha Shivaratri |
Some festivals add further conditions. Holika Dahan and Raksha Bandhan must avoid Bhadra (the Vishti karana that occupies the first half of Purnima), so they can move to the next day even when the tithi says otherwise.
Two traditions, two dates
Where communities genuinely follow different rules, there is no single correct date. Janmashtami is the clearest example: Smarta practice keeps the day on which Ashtami covers midnight, while Vaishnava sampradayas, ISKCON among them, keep the day on which Ashtami holds at sunrise — often the following day. This site computes both and lists both when they differ.
Month names differ too. Most of India counts the month from new moon to new moon (amanta); much of the north counts from full moon to full moon (purnimanta). The festival day is the same under both, but in the dark fortnight the month is named one month later under purnimanta.
The exceptions: solar festivals
Makar Sankranti and Mesha Sankranti (Baisakhi, Vishu, Puthandu) are fixed by the Sun entering a sidereal sign, not by the Moon. That is why they sit on nearly the same Gregorian date every year — 14 or 15 January, 13 or 14 April — while everything else moves.
A date here can differ by a day from a printed calendar for another city, because sunrise, midday and midnight are local. Choose your own city on the festival pages for the date that applies to you.