If Easter, like Christmas, is based on a specific event, why is the date not fixed and is instead based on the moon?
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Easter as a holiday has roots in the Jewish holiday of Passover. They do not celebrate the same thing, but the way they both became spring feasting holidays was not exactly coincidental. One of the effects of this is that Easter has its dates pegged to the Hebrew calendar, not the Gregorian one that the bulk of the world has standardized today.
The Gregorian calendar is a mostly pure solar calendar. It concerns itself with counting the days in the year, and nothing else. Every date on the calendar does its best to ensure that, on that day, no matter what year it is, it will always be the same season that day, and the sun will always be in the same spot in the sky on that day.
The primary counterpart to a solar calendar is a lunar calendar. Lunar calendars track moon phase, not sun position. They will often have months of 29-30 days synced up exactly with the moon cycles. It is wholly unconcerned with what season any given date is. So compared to a solar calendar, the dates will wildly drift apart. The Islamic calendar is a lunar calendar. It’s why Ramadan seems to be celebrated almost completely at random to someone used to a solar calendar. It’s because they’re counting moon phases, not season cycles.
The Hebrew calendar is lunisolar. A sort of compromise between the two. They still considered moon phase important, and track those with months synced to its phases. But they also still consider seasons important to track, for agricultural purposes. So their calendar ends up being sort of like the Islamic one, tracking the months and drifting out of sync with the solar calendar. But before they drift too far, they add a kind of “leap month” to get back in sync.
This slow drifting out of sync with occasional big-chunk corrections causes the Hebrew calendar to always be *roughly* in sync with the Gregorian calendar. Ususlly no more so than a month off. But every year they differ by a different amount. Since Easter is pegged to a Hebrew calendar date and not a Gregorian one, that’s exactly what happens to Easter when you try to mark it on a Gregorian one.
EDIT: To be a bit more specific, Easter is not exactly “pegged to the Hebrew calendar” either. It’s… a weird mix of Hebrew and Gregorian. A more precise definition of when Easter is celebrated is “the first Sunday after the first full moon after the spring equinox”. The “spring equinox” part is a day that all cultures, regardless of calendar system, were at least somewhat interested in. It’s one of only two days of the year when night and day are the same duration. Though, because that’s a solar event, only solar calendars track equinoxes on the same day every year. The “first full moon” part is where the Hebrew calendar comes in. They’re waiting for a particular date on their calendar, which is synced to the moon phases. Then, the “Sunday” part is like taking the Hebrew date, whatever it happens to be, and “rounding up to the nearest Sunday” in the Gregorian calendar, because people who use that calendar would prefer to celebrate this holiday on their designated holy day of the week. Going between solar to lunisolar and back to solar again in this way causes Easter’s date to “hop around” year-on-year… which is in no small part why the holiday became associated with rabbits.
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