Have you ever wondered why we sometimes celebrate Easter in March and then other times we celebrate it in April? Last year, Easter fell on Sunday (always a Sunday), April 21 and this year it falls on April 12.

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Why isn't Easter a fixed holiday like Christmas? Well, the easiest way to explain this is to tell you that Easter falls on the first Sunday after the first full moon which occurs on or after the vernal equinox.

It can be confusing but if we break it down a little more, it might make some sense. The vernal equinox is the first day of spring which usually happens on March 20 (this year it happens on Thursday, March 19). And you know what a full moon is, right?

According to the Bible, Jesus was resurrected around the time of the Jewish Passover, which was celebrated on the first full moon after the vernal equinox. Some Christians started celebrating Easter right on Passover, while other Christians waited for the following Sunday because that was the day of the week that the Resurrection occurred. However, in 325 A.D. at the Council of Nicaea, Emperor Constantine wanted everyone to celebrate Easter on the same date and so it was decided that the Sunday after the full moon should be the day to celebrate the resurrection of Jesus.

Christmas is always celebrated on December 25, but the date of Easter actually dictates several other Christian holidays throughout the year. Easter dictates the date of Palm Sunday (April 5 this year), Good Friday (April 10 this year), Ascension Day (May 21 this year), and Pentecost (May 31 this year).

[via Time and Date/CBS]