If you look at a medieval calendar, you'll see a column containing a seemingly random series of Roman numerals. These actually represent the dates of new moons. Like many things in the early calendar, the values are based on a theoretical value rather than the actual astronomical event.
The Roman numerals represent the Golden Number for the year. The Golden Number is based on the 19 year Metonic cycle where the solar year and the lunar year coincide. To find the Golden Number for a given year, divide the year by 19, and add one to the remainder. So for 2012, 2012/19 gives 105 with a remainder of 17. Adding one to that give a Golden Number of 18. Therefore everywhere you see 'XVIII' on the medieval calendar, that would be the date of the calculated new moon. Today (24 March) is next to XVIII on my re-created medieval-style calendar (see my medievalscience.org website under 'Time' for more information).
This new moon is special in the Church calendar. The new moon that falls between March 8 and April 5 (inclusive) heralds the Paschal Full Moon - the first full moon following the Vernal (Spring) Equinox. The full moon is defined as occurring 15 days (Roman inclusive counting we'd say 14 days using modern counting) after the new moon. The first Sunday following the the Paschal Full Moon is Easter. So counting forward from today, the Paschal Full Moon occurs on April 7, making April 8 Easter.
Aren't you glad all you have to do to find the date of Easter is look at a calendar?
Magister Galenus Ockhamnesis
Friar Galen of Ockham, OP
For article references, see links below.