Why does Easter's date change annually?

Easter's date changes annually because it is a "movable feast" determined by a complex interaction between the solar calendar and lunar cycles, specifically falling on the first Sunday after the first full moon occurring on or after the spring equinox (March 21) [https://www.lohud.com/story/news/2026/04/02/why-does-easter-date-change-every-year-explained/89435353007/]. Because lunar cycles do not align perfectly with the standard 365-day solar year, this calculation results in Easter shifting each year within a range of March 22 to April 25, creating a unique scheduling pattern that influences ecclesiastical, cultural, and commercial activities globally.

### What is the historical origin of this calculation?
The determination of Easter’s date dates back to the Council of Nicaea in 325 AD. Church leaders sought to create a standardized way to observe the holiday that remained consistent with its historical roots in the Passover tradition, which is tied to the lunar calendar [https://gracechurch.us/articles/what-determines-the-date-of-easter]. By choosing to link the resurrection celebration to the first full moon following the spring equinox, the church established a system that honors both the celestial markers and the theological timing of the event.

### Why do Western and Eastern churches sometimes celebrate on different dates?
The variation often stems from the use of different calendars. Most Western churches utilize the Gregorian calendar, whereas many Eastern Orthodox churches follow the older Julian calendar for calculating the date of Pascha (Easter) [https://www.lohud.com/story/news/2026/04/02/why-does-easter-date-change-every-year-explained/89435353007/]. These two calendars have drifted apart over centuries of history, meaning the calculation for the "first full moon after the equinox" frequently lands on different dates in the two traditions, occasionally resulting in the observances being weeks apart.

### How does the lunar cycle impact the calendar mismatch?
The fundamental issue is the discrepancy between the lunar month and the solar year. A lunar cycle, or the time it takes for the moon to complete its phases, is approximately 29.5 days [https://www.wdsu.com/article/easter-holiday-dates-explainer/70920377]. Our standard solar calendar is divided into months of 28 to 31 days to accommodate the 365-day year. Because these two cycles do not synchronize, the "Paschal Full Moon" moves across different dates in our standard calendar, forcing the associated Sunday to shift accordingly.

### Key Takeaways
* **Moveable Nature:** Easter is not fixed to a date but is determined by lunar and solar celestial events.
* **The Equinox Factor:** The calculation always relies on the spring equinox (March 21) as the baseline.
* **Ecclesiastical Variance:** Differences between the Gregorian and Julian calendars result in varying dates between Western and Eastern Orthodox traditions.
* **Predictable Range:** While the date changes, it is mathematically constrained to fall strictly between March 22 and April 25.

Understanding the mechanics behind Easter’s date serves as a fascinating intersection of astronomy, history, and religious tradition. While it may occasionally cause confusion for travelers or those planning seasonal events, this ancient calculation remains a testament to how early societies relied on the natural rhythms of the universe to anchor their most important cultural milestones. As technology and modern calendars dominate our lives, this tradition continues to remind us of the enduring legacy of the celestial observations that guided our ancestors.

## References
* [Why does Easter change every year? Here's how the date is decided - Lohud](https://www.lohud.com/story/news/2026/04/02/why-does-easter-date-change-every-year-explained/89435353007/)
* [Easter holiday: More on how the dates fall each year - WDSU](https://www.wdsu.com/article/easter-holiday-dates-explainer/70920377)
* [What determines the date of Easter? - Grace Church](https://gracechurch.us/articles/what-determines-the-date-of-easter)