[Some people] have difficulty with the Nicene Fathers’ setup for determining the Easter date (first Sunday after first full moon after March 21/vernal equinox) because the method of dating deviates from the Jewish calendar and its reckoning of the Passover days. Why did the Church abandon the regular dates for Passover to coincide with Easter? That seems like the most reasonable time to celebrate the Resurrection: during the Passover season.
Wikipedia’s article on the First Nicene Council has
a section on this issue. Wikipedia has no authority by itself, but one can cite it for its sources, and in this case it cites some very interesting primary documents on this point. Three sentences related to your question are these:
“[Some Christians] justified this break with [Jewish] tradition by arguing that it was in fact the contemporary Jewish calendar that had broken with tradition by ignoring the equinox, and that in former times the 14th of Nisan had never preceded the equinox.”
And: “[Some early Christians] argued that contemporary Jews were identifying the wrong lunar month as the month of Nisan, choosing a month whose 14th day fell before the spring equinox.”
And: “[Some Christians]…[did] their own computations to determine which month should be styled Nisan, setting Easter within this independently computed, Christian Nisan, which…always locate[d] the festival after the equinox.”
One of the footnotes it cites in this section is from Anatolius of Laodicea, a bishop from the 200s who made some of the arguments mentioned in the Wikipedia article. Relevant comments from him are preserved in
Eusebius’s History of the Church History Book 7 Chapter 32 beginning at paragraph 14.
He says, for example: “we maintain that those who place the [month of Nisan] in [March], and determine by it the fourteenth of the passover, commit [a] blunder.”
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And: “this is not an opinion of our own; but it was known to the Jews of old, even before Christ, and was carefully observed by them. This may be learned from what is said by Philo, Josephus, and Musæus; and not only by them, but also by those yet more ancient, the two Agathobuli, surnamed ‘Masters,’ and the famous Aristobulus…”
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This early bishop also created a table of Easter dates for many years into the future which were very influential.
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