it was probably orignally celebrated on a Monday, but the bishops gave up on making it a holy day of obligation so they moved it to Sunday so more Catholics would celebrate it.
seriously, folks, this is a non-issue, just like the date for Christmas. do you always have your birthday party exactly on your birthdate? don’t we move Federal holidays around like checkers on a board to have more long weekends? the NT is not dateline NBC or a minute by minute account of the events of the Life of Christ, still less is it intended as a literal timeline of the events immediately preceding and surrounding the Passion, Death and Resurrection. Read it as it was intended, a narrative, a synthesis, and in John, an epic poem, written by and for people in another culture and time sense, not as a Time mag cover story written to modern journalistic standards, still less as police evidence compiled by detectives reconstructing a crime.
Bible literalists, who persist in their fundamental error of ignoring the spiritual sense of Scripture in their slavish insistence on such details, deny themselves the fullness of God’s revelation. What a pity. If literal adherence to the actual date and sequence of events was so important the Church would have reconstructed painstakingly the actual dates of the events and Easter would be the same date every year. The importance of the timing of events in the Gospel narratives is making the link with Passover and the new Pasch, not in journalistic details. The Gospel abounds in such details, but they are always the details critical for illuminating the meaning of events, not peripherals.
