Easter Sunday typically falls on a different date each year. Easter is always the first Sunday after or on the first full moon, after the Spring (vernal) Equinox in the Northern hemisphere and the Autumnal Equinox in the Southern hemisphere.
The date is determined by a combination of events centered around the lunar cycle, the solar cycle, the division of each year into 365 days and a 1,700 year old Church ruling.
The explanation starts with the fact that early Christians elected to link the date of Easter to the Hebrew calendar. The New Testament states that the Resurrection took place on the first day of the week following Passover. Sunday is the first day of a Jewish week; the Passover falls on the day of the first full moon after the Spring Equinox, which can fall on either March 20 or 21.
Chaotic, or what? The result was that different churches ended up celebrating Easter on various days. And to try to clear up the confusion, the Roman Emperor Constantine I organized a major summit meeting.
The first Ecumenical Council was held at Nicea in present-day Turkey in the year 325. It decreed that Easter would be celebrated on the Sunday following the first full moon that occurred after the Spring Equinox. This retained a lunar connection as a sort of “memory” of the Jewish calendar system, and ensured that the feast would be on a Sunday. Because lunar phases occur independently of the solar year, this means that there is a “window” of several weeks during which Easter may be celebrated. By this reckoning, in our calendar, Easter must occur between March 22 and April 25.
So, go to your calendar, and find the first day of spring – then find the first full moon immediately following that. And Easter will be the first Sunday after (or on) that first full moon. Note the difference in the southern hemisphere: Easter falls on the first Sunday on or after the first full moon after the Autumnal equinox, not after the first day of Autumn.
The system that was slowly developed throughout the Middle Ages is the base for what we use today.