Lunar vs Solar · Why Ramadan and Easter Move
TLDR: A lunar calendar tracks moon phases (29.5 days per cycle, ~354 days per year). A solar calendar tracks the Earth’s orbit around the Sun (365.25 days). When holidays follow the moon, their date in the standard solar (Gregorian) calendar slips year-to-year. Easter is computed from a hybrid lunisolar rule; Ramadan is purely lunar; Diwali is lunisolar.
If you have ever wondered why Easter Sunday lands on a different date every year while Christmas always falls on December 25, the answer is calendar arithmetic. Some holidays are anchored to the Sun, some to the Moon, and a few to a careful combination of both. This article explains the math and shows you how to drill the dates with our Red Day Match game.
Solar calendars: anchored to the Sun
A solar calendar measures one year as the time the Earth takes to orbit the Sun · about 365.25 days. The Gregorian calendar (introduced in 1582) approximates this with 365 days plus a leap day every four years (with century rules to keep the average accurate). Christmas, New Year, and most national days are fixed Gregorian dates · they always fall on the same day of the year.
Examples of fixed solar dates: Christmas (December 25), US Independence Day (July 4), Bastille Day (July 14), New Year (January 1). Same Gregorian date every year, regardless of the moon.
Lunar calendars: anchored to the Moon
A lunar calendar measures one month as one phase cycle of the moon · about 29.5 days. Twelve such months come to about 354 days · 11 days short of a solar year. Pure lunar calendars (the Islamic Hijri calendar) make no attempt to correct for this, so lunar holidays drift backwards through the solar year by about 11 days each year.
The Islamic year is purely lunar. Ramadan starts on the 9th month of the Hijri calendar, which means in the Gregorian calendar Ramadan starts about 11 days earlier than the previous year. Over a 33-year cycle, Ramadan moves through every solar season · Ramadan in summer, then spring, winter, autumn, and back to summer.
Lunisolar calendars: a careful combination
A lunisolar calendar uses lunar months but periodically adds a 13th “leap month” to keep the calendar aligned with the solar year. The Hebrew calendar, the Hindu calendar, and the Chinese calendar all use this technique.
The leap-month rule keeps the average year close to 365.25 days, so Hebrew and Hindu holidays do drift through the solar year, but only within a window of about 30 days · they don’t loop the way Ramadan does.
Key distinction: Pure lunar holidays (Ramadan, Eid) drift through every season over decades. Lunisolar holidays (Passover, Diwali, Lunar New Year) drift within a 30-day window of the solar year but stay roughly in the same season.
Easter: a hybrid rule
Easter is the most-mathematically-elaborate moving holiday in the western calendar. The rule, set by the Council of Nicaea in 325 CE, is:
Easter Sunday is the first Sunday after the first ecclesiastical full moon on or after the spring equinox.
The “ecclesiastical” full moon is computed from a fixed table (the computus), not from astronomical observation, so the church and astronomical full moons can differ by a day or two. The result: Easter falls between March 22 and April 25 in the Gregorian calendar.
Eastern Orthodox churches use the same rule but apply it to the Julian calendar (which currently runs 13 days behind the Gregorian), so Orthodox Easter usually falls a week or more later than Western Easter.
Holidays computed from Easter: Good Friday (Friday before Easter Sunday), Ascension Day (40 days after Easter), Pentecost (50 days after Easter). These all share Easter’s lunisolar drift.
Why is Easter computed from the spring equinox? The earliest Christian congregations celebrated the Resurrection during Passover, which is itself anchored to the spring full moon (15 Nisan in the Hebrew calendar). The Council of Nicaea formalised the rule in 325 CE so all churches would celebrate on the same Sunday rather than splitting between Roman and Jewish dating conventions.
Astronomy footnote: “First full moon on or after the spring equinox” sounds simple but the church uses an ecclesiastical equinox (fixed at March 21) and an ecclesiastical full moon (computed from a 19-year metonic cycle). Astronomers and the church can disagree by a day, which is why occasionally astronomical Easter and ecclesiastical Easter differ.
Drill the moving dates
Recognising which holiday is fixed vs moving is the first step to learning the calendar. The game below presents a holiday from our library and asks for its date, religion, or country · play a few rounds with the filter set to a single tradition to master that calendar.
A common mistake: assuming Lunar New Year falls on the new moon nearest January 1. It doesn’t. Lunar New Year is the second new moon after the winter solstice (Dec 21), so it lands between January 21 and February 20.
Calendar systems summary
| Tradition | Calendar type | Year length | Drift behaviour |
|---|---|---|---|
| Western Christian | Gregorian (solar) for fixed feasts; lunisolar rule for Easter | 365.25 days | Easter window: Mar 22 – Apr 25 |
| Islamic | Pure lunar (Hijri) | ~354 days | Drifts backward 11 days/year, full cycle in 33 years |
| Hebrew | Lunisolar | ~365 days (leap month every ~3 years) | 30-day window inside Gregorian |
| Hindu | Lunisolar (regional variations) | ~365 days | 30-day window inside Gregorian |
| Chinese | Lunisolar | ~365 days | 30-day window inside Gregorian |
| Buddhist (Theravada) | Lunisolar | ~365 days | Tied to full-moon dates of specific months |
| Civic / Secular | Gregorian (solar) | 365.25 days | Fixed |
Rule of thumb: if a holiday has the same date every year, it’s solar. If it slides 11 days a year, it’s pure lunar. If it stays in the same season but moves up to a month, it’s lunisolar.
See also
- The History of Red Days · why public holidays print in red.
- Global Work Cultures: Bank Holidays across 50 Countries · how working calendars vary worldwide.
- Red Day Match game · drill the dates yourself.
Master the rule once and you’ll never need to look up Easter or Ramadan again · you’ll know how to compute it.
Red Day Match
Match public holidays to their date, religion, or country. Filter by Christian, Muslim, Jewish, Hindu, or Secular
Play nowWorks on any device.
