Easters exact date varies so much because it actually depends on the moon. The holiday is set to coincide with the first Sunday after the Paschal Full Moon, the first full moon after the vernal equinox. Because the Jewish calendar is tied to solar and lunar cycles, the dates of Passover and Easter fluctuate each year.

Why is Easter a movable feast?

Easter is a moveable feast celebrated on the first Sunday after the vernal equinox full moon (i.e. on Sunday after the full moon of March). These dates are calculated with the old Gregorian calendar, because it specifies the vernal equinox. The calculation of the Easter date is a complex mathematical problem.