My 'Conspiracy Theory' that the Roman Imperial Government Changed the Christian Veneration of the Eighth Day of Passover as the Proper Time for Baptism is Witnessed in the Fourth Century

For they [the Audians] choose to celebrate the Passover with the jews - that is, they contentiously celebrate the Passover at the same time that the Jews are holding their Festival of Unleavened Bread. And indeed, that this used to be the church's custom - even though they tell churchmen a slanderous thing in this regard and say "You abandoned the fathers' Paschal rite in Constantine's time from deference to the emperor, and changed the day to suit the emperor." And some, again, declare with a contentiousness of their own, "You changed the Passover to Constantine's birthday"

And if the Passover were celebrated on the same day each year, and it had been decided to keep it on that day at the council convoked by Constantine, what they say might be plausible. But since the rite cannot fall on the same day each year, their argument is worthless. The emperor was not concerned for his birthday, but for the unity of the church. In fact God accomplished two highly important things through Constantine, the most beloved of God and forever the most blessed. [One was] the gathering of an ecumenical council, and the publication of the creed that was issued at Nicaea and confessed the assembled bishops with their signatures - the deposition of Arius and the declaration to all of the purity of the faith. [The other was] their correction of the Passover for our unity's sake.

For long ago, even from the earliest days, the Passover was celebrated at different times in the church...[Epiphanius Vol. 2, pp.410-411; Book III, Heresy 70, chapter 9] (from tertullian.org)