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

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

Still not following me? It can be confusing, I know. However, maybe it'll help if I break it down a little more. You see, the vernal equinox is the first day of spring which usually happens on March 20th (this year it on was March 20th). 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's 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 25th, but the date of Easter actually dictates several other Christian holidays throughout the year. The date that Palm Sunday, Good Friday, Ascension Day, and Pentecost fall on all depends on the date of Easter.

[via Time and Date/CBS]