Many other holidays also occur on fixed dates. The only apparent constant for the Easter date is that it is always on Sunday. The Sun fixes the dates for immovable feasts such as Christmas, while the Moon determines the moveable feasts, such as Easter.
Many European countries call Easter Pascha.
This word comes from the Hebrew word Pesach, which means Passover. The council of Nicaea, in A.D. 325, refined the method for determining the date of Easter based on the time of the Paschal Full Moon. The date of the full moon varies each month because the interval between full moons is 29.5 days. According to the Book of Common Prayer , the Rule for Easter states that the celebration of Easter should always take place on the Sunday which immediately follows the Full Moon that happens upon, or next after, the day of the Vernal Equinox. The first day of spring in the northern hemisphere is the Vernal Equinox, and it moves between March 19 and March 21.