Why Diwali’s Date Changes Every Year

In October 2025, Diwali split down the middle. Cities where the sun set before 5:30 p.m. lit their lamps on October 21. Everywhere else celebrated a day earlier, on October 20. Same festival, same religious significance, nearly a billion people worldwide, and no single date that all of them agreed on. That split wasn’t a scheduling error or a regional dispute. It was the direct, predictable result of how Diwali’s date actually gets calculated, a method that has nothing to do with the Gregorian calendar most of the world uses to plan everything else.

Diwali moves. It has landed as early as mid-October and as late as mid-November, and it shifts by roughly eleven days most years before an occasional correction pulls it back. Understanding why requires setting aside the assumption that a calendar date is a fixed, unambiguous thing. For Diwali, it isn’t.

The Rule: The New Moon of Kartik, Not a Fixed Day

Diwali’s main day falls on Amavasya, the new moon, of Kartik, a month in the Hindu lunisolar calendar. That’s the entire rule. There’s no equivalent of “the fourth Thursday in November” or “December 25” involved anywhere in the calculation. The date is anchored to a specific phase of the Moon within a specific lunar month, and that lunar month has no fixed relationship to any Gregorian date at all.

The Hindu calendar is lunisolar, meaning it tracks lunar months, roughly 29.5 days each, while periodically correcting itself to stay aligned with the solar year of about 365.25 days. Twelve lunar months add up to about 354 days, eleven days short of a full solar year. Left uncorrected, lunar-based festivals would drift through every season over a few decades, the way the Islamic calendar’s Ramadan does, arriving roughly eleven days earlier each year with no seasonal anchor at all, a drift explored in the piece on Ramadan’s shifting dates. Diwali doesn’t drift that freely because the Hindu calendar corrects for the gap.

Adhik Maas: The Leap Month That Resets the Clock

To correct that roughly eleven-day annual drift, the Hindu calendar periodically inserts an entire extra lunar month, called Adhik Maas, roughly every two to three years. Rather than adding a single day the way the Gregorian calendar adds February 29 in a leap year, this system adds an entire 29-to-30-day month, which resets the accumulated drift in one move rather than correcting it gradually. This is why Diwali doesn’t simply migrate steadily through the seasons the way Ramadan does. It oscillates within a bounded window, generally mid-October to mid-November, drifting earlier for two or three years and then jumping back later once an Adhik Maas is inserted somewhere in the preceding cycle.

The four-year spread there, from October 20 to November 12, illustrates the range Diwali moves through even without a dramatic calendar shift. That’s the same underlying mechanism, a lunisolar calendar reconciling itself against the solar year, that governs Chinese New Year’s shifting date, though the two calendars use different rules for where exactly their leap months go and which lunar month anchors the main festival.

Why a Single New Moon Can Split a Country in Two

The 2025 October 20 versus 21 split wasn’t a disagreement about the rule, everyone agreed Diwali falls on Kartik Amavasya. It was a disagreement about which civil day the relevant new moon actually belonged to, and that turns out to be a genuinely harder question than it sounds.

A tithi, the lunar day used throughout the Hindu calendar, isn’t a fixed 24-hour unit the way a solar day is. It’s defined by the angular distance between the Sun and Moon increasing by exactly 12 degrees, and because the Moon’s orbital speed varies, a single tithi can run anywhere from about 19 to 26 hours. That means a tithi’s start and end times shift relative to sunrise and sunset from month to month, and it can straddle two different Gregorian calendar days entirely.

In 2025, the Amavasya tithi began at 3:44 p.m. on October 20 and ran until 5:54 p.m. on October 21, present at sunset on both of those days in most locations. Traditional guidance holds that when a tithi stays present for at least 24 minutes past sunset on two consecutive days, the festival is observed on the second of the two. Cities with an earlier sunset, at or before roughly 5:30 p.m., still had a comfortable 24-minute cushion of Amavasya left after sunset on October 21, so they celebrated that day. Cities with a later sunset found Amavasya had already expired too close to, or before, their own sunset on the 21st to clear that threshold, so the rule fell back to October 20 instead, when the new moon was unambiguously present at twilight. Same astronomical event, two different local answers.

Five Days, Not One

Diwali itself, the Lakshmi Puja night, is the third of five consecutive days, each with its own name, mythology, and rituals.

The whole five-day arc moves together with the main Amavasya night, since every other day is defined relative to it on the lunar calendar rather than fixed independently. The night itself sits on the darkest phase of the lunar cycle, which is part of why it’s treated as the moment for lighting lamps against the dark.

One Night, Three Different Origin Stories

For Hindus, the main night commemorates Rama’s return to Ayodhya after 14 years of exile and his defeat of the demon king Ravana, with residents said to have lit rows of oil lamps to welcome him home, the origin of Diwali’s name, a row of lights. The same night also carries older associations with the churning of the cosmic ocean and the emergence of Lakshmi, goddess of wealth, which is why Lakshmi Puja is the central ritual of the evening regardless of which mythological account a household follows.

Jains mark the same night as Mahavira Nirvana Divas, the anniversary of Mahavira, the last Tirthankara of the current era, attaining moksha, or final liberation. Sikhs observe it as Bandi Chhor Divas, commemorating the sixth Sikh Guru, Guru Hargobind, being freed from imprisonment at Gwalior Fort in 1619 along with 52 other detained princes, and the Golden Temple in Amritsar is illuminated for the occasion. Three unrelated historical and spiritual events, from three distinct traditions, converging on the exact same new moon night is not a coincidence built into the festival’s design so much as centuries of independent traditions accumulating around the same astronomically significant date, the darkest night of the lunar month.

North and South Don’t Always Agree on Which Day Matters Most

Even once a region has settled which Gregorian date corresponds to Amavasya, there’s a further layer of regional variation in which day of the five gets treated as most important. In much of North India, the Lakshmi Puja night itself, Amavasya, is unambiguously the main event. In large parts of Tamil Nadu, Telangana, Karnataka, and Andhra Pradesh, Naraka Chaturdashi, the day before, is treated as the principal celebration, observed with a pre-dawn ritual oil bath and centered on Krishna’s victory over Narakasura rather than the Lakshmi Puja that follows that evening. Neither tradition disputes when the new moon falls. They simply place the weight of the festival on a different one of the five days, a distinction that traces back through centuries of parallel regional practice rather than any disagreement over lunar astronomy.

Diwali Enters American Law

The clearest sign of how far Diwali’s reach has grown outside India shows up in recent legislation, not religious texts. In 2023, New York Governor Kathy Hochul signed a law making Diwali a mandatory public school holiday across New York City, replacing an older, little-observed holiday on the school calendar. Because the date moves every year, the law doesn’t name a fixed day; it ties the closure to whichever civil date corresponds to Kartik Amavasya each year, the same lunar calculation described above, meaning school administrators effectively have to consult a Hindu Panchang, not just a fixed almanac, when building each year’s calendar.

California went further. Assembly Bill 268, authored by Assemblymembers Ash Kalra and Darshana Patel, made Diwali an official state holiday, explicitly authorizing school closures and paid leave for state employees, and takes effect for the first time in 2026, when the main celebration falls on Sunday, November 8, meaning California’s state workers will observe their first official Diwali holiday on Monday, November 9. California has the largest Indian American population of any U.S. state, and the law’s passage reflects a broader pattern: institutions built entirely around the fixed rhythm of the Gregorian calendar are increasingly having to build in room for a festival that refuses to sit still on it, the same accommodation long since made for Easter’s moving date and other movable observances tied to lunar rather than solar timing.