Paschal full moon

Theoretically, the Paschal full moon is the first full moon occurring on or after the spring equinox.

When is easter

However, this day can be reckoned in different ways. One way is by looking at the sky, which yields the astronomical spring equinox. But since this shifts from year to year, most people follow the calendrical spring equinox, which is reckoned as March 21.

On the Gregorian calendar (the one that we use), Easter is the first Sunday after the Paschal full moon, which is the first full moon on or after March 21.

Easter thus always falls between March 22 and April 25.

Now, to find Palm Sunday (the sixth Sunday of Lent) you start with the date of Easter and back up one week: It is the Sunday before Easter Sunday.

To find Ash Wednesday, you start with the date of Easter Sunday, back up six weeks (that gives you the first Sunday of Lent), and then back up four more days: Ash Wednesday is the Wednesday before the first Sunday of Lent.