Tukuk-OSThe Fixed Half: A Calendar That Matches the Sun Almost Exactly
The Gregorian calendar we all use for work and school is a solar machine: it hems the 365.2422-day Earth orbit with a leap day every four years minus three exceptions per century. Because the calendar tracks the same cycle the seasons do, solstice-anchored holidays sit on fixed dates. Christmas on 25 December, Nowruz at the spring equinox, Independence Days on their historical dates — the year wraps neatly around them. Any country's fixed holidays can be printed once and pasted on a wall forever, which is why so many national dates on tukuk.org/holidays are perfectly still while their religious neighbours drift.
The Drifting Half: Calendars That Only Watch the Moon
The Islamic calendar is purely lunar: twelve months of 29 or 30 days, about 354 days a year — eleven days shorter than the solar year. Nothing forces it back into step, so Ramadan, Eid al-Fitr and Eid al-Adha walk backwards through the seasons, completing the circuit in roughly 33 years. That is a feature, not a bug: Muslim scholars note it eventually distributes the fasting hardship across every climate and every working season. The Hebrew calendar is lunisolar — lunar months, but with a thirteenth month inserted seven times every nineteen years so Passover stays in spring. The Chinese calendar does similar intercalation, which is why Chinese New Year floats between 21 January and 20 February. Diwali follows the Hindu lunisolar month of Kartik and lands between mid-October and mid-November. The pattern is one idea repeated: months are moons, and the leftover fractions of a solar year are paid back with the occasional bonus month.
Easter: The Most Complicated Date in Business
Easter obeys a rule written in 325: the Sunday after the first full moon on or after the 21 March ecclesiastical spring equinox. Note the word ecclesiastical — the church uses a tabulated, idealised moon, not the one in the sky, so Easter can theoretically disagree with an actual astronomical full moon. The result is a date that can fall anywhere between 22 March and 25 April, and that drags a whole family of movable feasts — Ash Wednesday, Palm Sunday, Ascension, Pentecost — along with it, which is why school terms and carnival seasons in many countries are permanently tied to one wandering Sunday.
Why the Mix Is a Political Achievement
Keeping church holidays fixed while the market calendar runs on the Gregorian year, while lunar-calendar communities keep their own counts, is precisely what the modern state manages — the same pragmatic compromise behind time zones, where nature's noon had to be squared with the railway timetable. Every country resolves it differently, and every answer is printed in law: a holiday list is a national policy in disguise — which religions get a public day, which get a floating one, how many days workers actually rest, and how much tourism the fixed dates were designed to feed. The next time your diary offers a long weekend, you can trace it back to an orbit, a phase of the moon, and a committee — usually all three.