On April 21, Christians will be celebrating Easter, the day on which the resurrection of Jesus is said to have taken place. The date of celebration changes from year to year.
The reason for this variation is that Easter always falls on the first Sunday after the first full moon following the spring equinox. So, in 2020, Easter will be celebrated on April 12, and on April 4 in 2021.
I am a religious studies scholar specializing in early Christianity, and my research shows that this dating of Easter goes back to the complicated origins of this holiday and how it has evolved over the centuries.