It was. THE EASTER VIGIL!
And where do you get the notion that the Church cannot make changes to worship schedules?
Holy Days of Obligation have been added, and deleted, and combined, and moved to Sundays, throughout history. Laws of fast and abstinence have been changed, and the recitation of the Divine Office, long considered an obligation binding under pain of mortal sin for those in Orders, can be dispensed by a bishop for almost any reason.
From the Third Commandment, and the New Covenant through whose light it must be read. The Church is the sole and proper interpreter of Scripture, but even she cannot make it say what it does not say.
The Church can change all manner of Holy Days and related obligations except the Sabbath obligation.