Easter and the holidays that are related to it are moveable feasts, in that they do not fall on a fixed date in the Gregorian or Julian calendars (both of which follow the cycle of the sun and the seasons).
Instead, the date for Easter is determined on a lunisolar calendar similar to the Hebrew calendar.
The word originally denoted the Jewish festival known in English as Passover, commemorating the Jewish Exodus from slavery in Egypt.
applied the term to Christ, and it is unlikely that the Ephesian and Corinthian Christians were the first to hear Exodus 12 interpreted as speaking about the death of Jesus, not just about the Jewish Passover ritual.
Easter and the holidays that are related to it are moveable feasts which do not fall on a fixed date in the Gregorian or Julian calendars which follow only the cycle of the Sun; rather, its date is offset from the date of Passover and is therefore calculated based on a lunisolar calendar similar to the Hebrew calendar.
The First Council of Nicaea (325) established two rules, independence of the Jewish calendar and worldwide uniformity, which were the only rules for Easter explicitly laid down by the council.
Because of the thirteen-day difference between the calendars between 19, 21 March corresponds, during the 21st century, to 3 April in the Gregorian Calendar.No details for the computation were specified; these were worked out in practice, a process that took centuries and generated a number of controversies.