How to Calculate Easter

The way we calculate Easter uses both a solar and lunar calendar. The church would set the equinox that vernal or spring equinox on March 21, even though it can move around a little bit. In 1582, Pope Gregory says, let's fix this. So in 1582, he decrees, you're going to go to bed on October 4th. And when you wake up, it's October 15th.