I’m going to offer a slightly different version of the “Easter is not pagan; Easter is a Christian holiday” meme. It goes like this:
In AD 324, the emperor Constantine summoned the Christian bishops to a council at the summer palace in Nicaea. Nicacea was a nice summer holiday town on the Propontis or Sea of Mamara — good beaches, excellent food, great celebrity-watching, and so on. No film festival, but hey, you can’t go to the Byzantine Riviera in the early 4th century and expect everything to be hunky-dory.
According to the history compiled by Eusebius of Caesarea, who was there, and allegedly a close confidant of the Emperor and numerous of the bishops, the bishops discussed the question of a creed — a formal statement of what all Christians had to believe. And, with the Emperor’s prodding (and eventually, threats), the bishops agreed on the Nicene Creed. By about AD 450, the two surviving letters of St. Patrick, that guy in Ireland that we get so excited about in mid-March, use language from the Creed.
The Emperor wasn’t done yet. He made the gathered bishops settle a number of other matters, and required them to pass a series of canons or ecclesiastical laws to govern the behavior of Church officials. Among these canons is one that formally establishes the astronomical calculation for determining the date of Easter each year. This formula thus pre-dates the separation of the Roman Catholic and Orthodox churches by about 600 years; and it was a sticking point in the eventual union of the Celtic and Roman Catholic churches (finally settled at the Synod of Whitby in 664 AD).
The formula is this: “The date of Easter shall be the first Sunday after the first full moon after the Spring Equinox.” Eusebius of Caesarea, and other authors from the time of the Council of Nicaea, tell us that the Church fathers picked this formula to guarantee that Easter would be celebrated on a Sunday in accordance with the Emperor’s wish, and on a date that paid notice to the events surrounding the Crucifixion in the Gospels — namely that Jesus was crucified on a Friday just as the feast of the Passover was starting, that he was taken down before Sundown in respect to the Jewish custom to not leave criminals hanging on the Sabbath or on Passover (since both began at sundown that Friday), and that Mary first encountered the risen Jesus in the garden on Sunday morning. Eusebius further tells us that the Christian leadership at the time continue to have doubts about the wisdom of Jews and Christians worshipping together, and so the formula for dating Easter is made slightly different from the formula for finding the date of Passover (by using the Graeco-Roman secular calendar’s “day of the Sun” following a specific lunar event, rather than just holding Easter on the date of the full moon following the spring equinox [the same time as Passover].)
So. We can date the occasion on which Christians adopted a formal date of Easter (AD 325); we can note this formal adoption of the formulary for determining the date in both Imperial secular, and Christian sources; we can see that saint Patrick, writing in around AD 450, has accepted the system for calculating the date of Easter; we can see that not all Britons have agreed to that formulary by the fact that it’s finally agreed upon at the Synod of Whitby in AD 664.
And, we can see in the historical record that this calendrical formula is adopted in the face of, and under enormous pressure from, the actual sitting living Roman Emperor, while the Council is meeting in front of his throne in his living room. If the Nicaean fathers did steal the date of Easter from any extant pagan system of religion, they did so with the express advice, consent and even informal arm-twisting and perhaps even not-even-lightly-veiled-threat-of-imminent-death of the secular authorities.
But, there’s no evidence that they did steal the formula from any extant pagan system. Instead, there’s a fairly obvious rationale for the formula which they did settle on — a date close to, but not exactly the same as, the Jewish Passover, in order that the festival of Easter can closely align to but not be exactly the same as, the time period corresponding to the last days of Jesus’s life and ministry on earth before the Crucifixion. Using the best records they had, and the most historical reasoning they could, they picked out as scientific a date-fixing scheme as they could muster.
In return for rules, and a formal creed that could be universally applied, the Nicaean fathers got the full support of the Imperial court, access to land and property and titles, the closure of all temples of the Olympian and Chthonic gods in AD 381, and judicial and executive powers over their dioceses and the Christians living in them.
The city of Rome was sacked by barbarian armies in AD 410, and the last Roman emperor abdicated in AD 450. The system for calculating the date of Easter survived in both eastern and western Christianity until the Gregorian calendar reforms of the 1500s; a period of confusion followed in the West until finally even mostly-Protestant nations like Britain and the American colonies accepted the revisions, while Orthodox churches continue to use a range of calendrical systems based on the old Julian calendar for finding the date of Easter (and some have accepted the Gregorian reforms).
TL;DR? Yeah, Easter isn’t pagan.