Why Easter day is Special ?

Easter's date changes each year because it's a "moveable feast" linked to the lunar cycles and the spring equinox. It's celebrated on the first Sunday after the first full moon that occurs on or after the spring equinox (March 21), which falls between March 22 and April 25. This calculation, known as computus paschalis, is used by both the Gregorian and Julian calendars, although the Julian calendar is used by Eastern Orthodox churches, leading to a slightly later Easter date for them. 

Here's a more detailed explanation:

Lunisolar Calendar: Easter's date is tied to the lunar cycles, similar to the Hebrew calendar, rather than being fixed on the Gregorian or Julian calendars. 

Spring Equinox: The spring equinox (March 21) serves as the starting point for the calculation. 

First Full Moon: The first full moon after the spring equinox is then identified. 

First Sunday: Easter Sunday is celebrated on the first Sunday after that full moon. 

Julian Calendar Variation: Eastern Orthodox churches use the older Julian calendar, which has a 13-day difference with the Gregorian calendar, leading to a later Easter date for them.