to it. I refer to the argument over the dating of Easter. It passed through several stages, beginning with the so-called Quartodecimans (if you're a stickler for Latin grammar, Quartadecimans) of the late second century, who kept Easter with the Jews when they observed Passover, the fourteenth of the (lunar) month of Nisan. They were opposed by an apparent majority, who kept Easter on Sunday after observing the death of Jesus on a Friday. You can read about it in Book 5, sections 23-25, in the Ecclesiastical History of Eusebius of Caesarea, which includes pertinent (if selective) documentation. The issue was still unresolved at the time of the Council of Nicaea in 325, where it was alleged to be the other reason for the Council beyond the controversy associated with Arius, as you can read about in Eusebius' Life of Constantine. It was resolved to hold it on the first Sunday after the first full moon after the Vernal Equinox. Western Christians were still fighting over it when the Venerable Bede wrote his Ecclesiastical History of the English People in the early eighth century (too many places to mention). I pass over the Eastern Christians who have clung to the Julian calendar.

Edited for writing lapses. I should "pretty close" to earliest in the sense of issues that emerged outside the writing and canonization of the New Testament.

Easter is the first Sunday after the first full moon after the Vernal Equinox.

At least that's what I was always told.

the first full moon of spring, as you noted.

There are three main calendar systems in use. We're familiar with the solar calendar.

A lunar calendar, such as the various Islamic calendars, has 12 lunar months in its year of approximately 342 days. Each month of 28 or 29 days follows the same cycle of the moon - some based on local astronomy and some based on Mecca's moon appearance. The "short" year explains why the month of Ramadan drifts around on our solar calendar. Beside the drift of seasons a given month doesn't have the same number of days year to year and it's nearly impossible to calulate the number of days between two dates + your birthday comes around a bit faster.

The lunar-solar calendar, such as the Hebrew calendar averts some of these shortcomings by adding a lunar month during various year to keep it closer to the solar calendar.

Then, of course is the Ethiopian calendar. I think it's still Julian and the year is off by 8 years IIRC due to a disagreement when Christ was born

On our solar calendar, Independence Day is always celebrated on July 4.

On the Jewish lunar calendar, Passover always begins on Nissan 15.

That's why I think of Passover as being determined on an actual lunar calendar while Easter is lunar-related but not exactly the same thing.