Easter is a moveable feast. The date varies from year to year.
It is related to the Jewish Passover because that is allegedly the time that Jesus Christ was crucified.
Passover is celebrated at the first full moon after the Vernal Equinox.
It was decided at the Council of Nicea in the fourth century CE that Easter would be celebrated the first Sunday after the first full moon after the Vernal Equinox.
The Vernal Equinox is usually tagged as 21 March, though because of slight astronomical anomalies it can sometimes be 22 March. If the full moon is a Sunday, Easter is postponed for a week. These rules apply if the Gregorian calendar is observed.
Some Eastern Orthodox Christians still use the Julian calendar so this is why dates are different.
A trivial fact: In the UK the Easter Act (1928) is still on the statute database even though it was never implemented. If it had been then Easter would have been fixed as the first Sunday after the Second Saturday in April.
Another mystery solved