Easter is the Sunday following the Paschal Full Moon.By this reckoning, the date of Easter can occur as early as March 23rd and as late as April 25th.
But what is the Paschal Full Moon? It is close to the astronomical full moon after the Spring Equinox in the Northern Hemisphere, but is not always exactly the same and can vary by a day or two. (Remember that Spring in the Southern Hemisphere is not the same as in the Northern.)
The Paschal Full Moon is the full moon after the Ecclesiastical full moon, which is fixed as March 21st and relates more closely to a crescent moon than a full moon (largely determined from tables established by the Medievals based on lunar months [and remarkably accurate]), falling thirteen days prior to the Paschal Full Moon.
By following the Paschal Full Moon as opposed to the Astronomical Full Moon of the Spring Equinox, the Church is able to celebrate Eater at the same time in both hemispheres (clever, huh?).
Now, back to the question.
The most common answer given to the dating of Easter goes as follows:
Easter is the first Sunday after the first full moon after the Spring Equinox.While this definition may not be precisely correct, it is correct enough for our usage because Easter always falls this way in the Northern hemisphere. As such, I will continue to use it.