Simply put, Easter is the first Sunday after the first full moon after the vernal equinox (the fifteenth day of the lunar calendar). If the full moon falls on a Sunday, it will be delayed by one week.

For example, March 2 1 day in 2023 is the vernal equinox, the first full moon after the vernal equinox is on April 5 (the 15th day of the third lunar month), and the first Sunday after April 5 is on April 9, so Easter in 2023 is scheduled for April 9.

For another example, March 20, 20021year is the vernal equinox, and the first full moon after the vernal equinox is on March 27 (February 15 of the lunar calendar), which happens to be a Sunday, so Easter falls on the next Sunday, so Easter in 20021year is April 4.