Easter is a Christian holiday that celebrates the resurrection of Jesus Christ. It is observed on the first Sunday after the full moon that occurs on or after the vernal equinox (the first day of spring). The date of Easter is determined by a combination of two factors: the lunar cycle and the solar cycle. The lunar cycle determines when the full moon falls, and the solar cycle determines when the vernal equinox falls. By combining these two cycles, we can determine when Easter will fall each year.