How is the date for Easter determined? The date for Easter is calculated as the first Sunday after the first full moon after the vernal equinox. The vernal equinox is the day of the yearn in the Spring when the day and the night are of equal length.
We know that Easter must be on a Sunday since all of the gospels clearly indicate that Jesus rose from the dead on the first day of the week. We also know that Jesus was celebrating Passover during the Last Supper. So Easter must fall on a Sunday around Passover each year. Passover is on the first full moon after the vernal equinox.