“Easter” is the period of eight Sundays [until Pentecost], comprising fifty days, often called as a unit “the Great Fifty Days.” For the explosive force of the resurrection of the Lord is too vast to be contained within a celebration of one day.
Easter is not one closing day at the end of a lengthy period of Lent. Easter is one extended rejoicing in the resurrection that more than exceeds in length the Lenten disciplines. The first day of the season, Easter Day, is the opening of a protracted celebration, even as the Resurrection is itself the opening to a vast new reality.
“The First Sunday After Easter” implies Easier is over, having lasted only one day. But “the Second Sunday of Easter” (for the same date) indicates that Easter is an extended season, whose essential character is shared by all of its parts.
–-Laurence Stookey, Calendar: Christ’s Time for the Church, 54, 56-7