One of the great vexing questions of early Christianity concerned (believe it or not) when to celebrate Easter. N.S. Gill has done a nice job explaining the various positions. Wikipedia’s rundown is also pretty good.
But it’s best to engage the primary texts. So I hope Father Raniero Cantalamessa’s excellent Easter in the Early Church: An Anthology of Jewish and Early Christian Texts is atop your reading list for the season that begins next week.