The Council of Nicea (325 AD) fixed Easter on the first Sunday after the first full moon after the spring equinox (the first day of spring). It is based on the lunar not the solar calendar, which is why its date changes every year (see Catechism of the Catholic Church, 1170). Easter never occurs before March 22nd or after April 25th.
- The first eight days of the Easter season are known as the Octave of Easter.
- Ascension Thursday occurs 40 days after Easter.
- Pentecost is the 50th day after Easter and ends the Easter celebration.
- Each Sunday is a "little Easter." From biblical times, Sunday has been known as "the Lord's Day" (see Revelation 1:10) because Christ rose from the grave on a Sunday morning; its name in languages such as Spanish ("Domingo") still reflect that designation (See Catechism of the Catholic Church 1166).