Passage: 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 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. No details for the computation were specified; these were worked out in practice, a process that took centuries and generated a number of controversies. (See also Computus and Reform of the date of Easter.) In particular, the Council did not decree that Easter must fall on Sunday. This was already the practice almost everywhere.In Western Christianity, using the Gregorian calendar, Easter always falls on a Sunday between 22 March and 25 April inclusive, within about seven days after the astronomical full moon. The following day, Easter Monday, is a legal holiday in many countries with predominantly Christian traditions.

Question: What calendar is used to determine when Easter is?
Gregorian