Orthodox Easter and Catholic Easter are on different dates due to differences in the calculation of the date of Easter. The date of Easter is based on the Julian or Gregorian calendar, which are different calendars used in different parts of the world. The Catholic Church uses the Gregorian calendar, which was introduced in 1582 by Pope Gregory XIII, and Orthodox churches use the Julian calendar which is older. The calculation of the date also takes into account the first full moon after the vernal equinox. There is a 13-day difference between the Julian and Gregorian calendars, which means that sometimes Easter is celebrated on different dates by the Orthodox and Catholic churches. Additionally, some Orthodox churches follow the revised Julian calendar, which is a modified version of the Julian calendar, resulting in a closer date to the Catholic Easter.