Why was Eid in March this year but April last year? Why does Easter hop between March and April? The answer lies in lunar vs solar calendars — the two great systems humanity uses to track time. Understanding the difference explains why some holiday dates are fixed forever while others wander the calendar.
The solar calendar: fixed and predictable
The Gregorian calendar — the world’s civil standard — is solar: it tracks Earth’s orbit around the sun (about 365.25 days). Holidays tied to it never move. Christmas is always 25 December, New Year’s Day always 1 January, Independence Days stay put. The only wrinkle is the weekday, which shifts yearly, and the occasional “observed” Monday or Friday when a date lands on a weekend. If a holiday is solar, you can plan it decades ahead with total confidence.
The lunar calendar: drifting through the seasons
A lunar calendar tracks the moon’s phases — twelve moon cycles make about 354 days, roughly eleven days short of a solar year. The Islamic calendar is purely lunar, which is why Ramadan, Eid al-Fitr and Eid al-Adha arrive about eleven days earlier each Gregorian year, cycling through all four seasons over 33 years. That drift is exactly why our 2026 holiday lists mark Islamic dates as “expected”: the date depends on the actual moon sighting, confirmed by religious authorities just before.
The hybrids: lunisolar calendars
Many traditions split the difference with lunisolar calendars, which track the moon but add leap months to stay aligned with the sun’s seasons. The Hindu calendar (Diwali, Holi), the Hebrew calendar (Passover, Rosh Hashanah) and the Chinese calendar (Lunar New Year) all work this way — which is why Diwali stays in October/November and Holi in February/March rather than drifting through the year like Eid. Easter uses its own ancient formula (the computus), tied to the spring equinox and the full moon, landing anywhere from 22 March to 25 April.
What this means for your planning
The practical rule is simple. Solar holidays: book with confidence, years ahead. Lunisolar holidays (Diwali, Holi, Easter): the month is predictable, the exact date usually known well in advance — safe to plan around. Pure lunar holidays (Eid al-Fitr, Eid al-Adha, Ashura): treat published dates as expected, keep bookings flexible, and wait for the official moon-sighting announcement before locking anything non-refundable. That three-tier approach will save you from ever being caught out by a shifting date again.
Quick quiz: which calendar decides it?
Test the framework on real 2026 dates. Christmas Day, 25 December? Solar — fixed forever. Eid al-Fitr, expected 20–21 March? Pure lunar — moon sighting decides, and it drifts eleven days earlier each year. Diwali, 8 November? Lunisolar — the Hindu calendar’s leap months keep it in autumn, but the exact day follows the new moon. Easter Sunday, 5 April? Its own ancient formula, tied to the spring equinox and full moon. Chinese New Year? Lunisolar — always late January or February, never outside that window. Once you can sort any holiday into one of these buckets, you will never again be surprised by a date that “moved” — you will know exactly why it moved, and roughly where it is going next.