About 358 AD, Hillel II established a fixed Jewish calendar based on mathematical and astronomical calculations. Click here to access his actual dates for 563 AD and view approximate dates for 31 AD that are based on Pope Gregory's western calendar established in 1582.
Ten days were added to the calendar by Pope Gregory in 1582 when October 5th became October 15th. So to convert back to a previous Julian date, if it is just prior to that day in 1582 we subtract 10 days from the Gregorian date, if before 1500 9 days, 1400 8 days, 1300 7 days, 1100 6 days, 1000 5 days, 900 4 days, 700 3 days, and before 600 was 2 days.
Accordingly that Sunday in 563 AD is in fact March 25th in the Julian calendar that the Romans used and Saturday becomes March 24th.
Continuing with the above rules, if before 500 subtract 1 day from the date.
Before 300 AD, the 10 days are finally accounted for.
Then, if before March 1st in 200 AD, subtract 1 day from the day of the week instead, to account for the leap day that was used in the Julian calendar on Feb 29th.
and if before 100 AD, subtract 2 days from the day of the week.
Thus that Passover "Tuesday" in 31 AD lines up again as Sunday March 25th, with Preparation "Monday" being actually on Saturday March 24th.
Now, the actual time when it was chosen (as recorded in Scripture) was back then based on a priestly declaration regarding the new moon and the start of the month.
Accordingly, the early church records - according to Dionysius in 525 AD - that Easter came on Sunday March 25th in 31 AD - that the Passover Sabbath came on Saturday March 24th - and the Preparation / Crucifixion on Friday March 23rd - though it may suggest a one day discrepancy - the later change of approach means that there is minimal / no historical contradiction with Hillel's calendar.