The Easter Cycle is a chunk of the Roman Catholic (R.C.) Church year centered on Easter.  The Sundays in the Easter Cycle either count down to Easter, or use Easter as the beginning of a subsequent count.  The date of Easter depends on the date of the Paschal Full Moon, and that date changes from year to year on solar calendars.  So, the dates of Easter Cycle feasts also change from year to year on solar calendars.  Easter Cycle feasts are for that reason called movable.  The Church year also has a non-moving part, namely, the Christmas Cycle, which includes the seasons of Advent, Christmas, Epiphany, and Time after Epiphany.  The Christmas Cycle is centered on the date of Christmas Day, Dec. 25.  In solar calendars, that is a fixed date.

The Christmas Cycle and the Easter Cycle together span one entire year.  The Easter Cycle shifts its position from year to year, causing changes to the beginning and end of the Christmas Cycle.

WHAT IS THE PASCHAL MOON, AND WHY USE IT?

Circa AD 30, Our Lord was crucified on the eve of a Sabbath day, on the first day of a Passover, and on the day of a full Moon.  His Resurrection was then on the first day of the week after the Sabbath, still during the days of Passover, and after a full Moon.  The early Church desired to keep those specific attributes in connection with the observance of the Feast of the Resurrection, and today’s Church still endeavors to do so.  Easter was traditionally observed either on Passover, or on the Sunday after Passover.  And the date of Passover is determined partly by lunations, not strictly by a solar calendar.  According to the USNO, the date of Passover is determined by the Jewish calendar, which is lunisolar (That info was from the USNO Date of Passover page, which is currently, “404 Not Found”).  The R.C. Church no longer follows the Jewish calendar, but uses a solar calendar (the Gregorian), and calculates the date of the Paschal (or Passover) Moon differently.  According to the USNO, the R.C. Church defines the Paschal Moon to be the first full Moon on or after March 21, and full is defined as the 14th day of the new moon.  The date of Easter is the first Sunday after the Paschal Full Moon.

In some years, the Jewish Passover Moon varies by one lunation from the R.C. Paschal Moon.  And either of those may vary from the Julian calendar’s Paschal Moon (which some Churches still use).  However, all these full Moons occur near the vernal equinox.  And the Moon is not full on the same dates year after year.  The lunar year consists of 12 lunations (or 354.37 mean solar days).  The solar year consists of about 365.25 mean solar days.  So, when an event depending on the date of a full Moon is placed in a solar calendar, it cannot occur on the same solar date from year to year.  That is why (from the Gregorian calendar frame of reference) the date of the Jewish Passover changes from year to year, as does the date of Easter.  Now let’s consider the movement of the date of Easter relative to the lunar calendar.

March 21 is an earth orbit (solar calendar) event, not a lunar orbit event.  Thus, the ecclesial calendar’s Paschal Moon is not just one lunar calendar Moon.  As soon as the Paschal Moon in use slips too far away from the solar calendar date of March 21, the calendar drops that Moon, and uses the next Moon in the lunar year for the Paschal Full Moon.  If the Paschal Moon did not change like that, that is, if Easter was affixed or pegged to just one lunar calendar Moon, then the Easter Cycle would crash its way through the fixed dates and observances of the entire solar year.  It would do that in Metonic (19 year) cycles.  Defining the Paschal Full Moon to be the first full moon on or after March 21 limits the shifting of the date of Easter to a maximum of the 34 days between March 22 and April 25 (That info was from the USNO Date of Easter page, which is currently, “404 Not Found”).

The use of a Paschal Moon is important, not just because the ecclesial calendar thus preserves the connection of the Resurrection to Passover, but also because each full Moon is a counter; the count of full Moons since the Crucifixion serves as a valid and accurate reckoning of New Covenant times.  These began not with the birth of Jesus, but with His institution of the Holy Eucharist and His Crucifixion.  The world’s scholars do not know the exact year of Christ’s Crucifixion.  Thus they do not know the exact lunation count.  That ignorance, however, is because of the negligence of men.  The Moon has been keeping time faithfully over the millennia.  And God is able to reveal to someone the number of lunations since the Crucifixion.  From that, the AD and the solar month of the Crucifixion could be accurately calculated.

NOTES:

The R.C. Church implemented the Gregorian calendar Oct 15, AD 1582, but the calendar was accepted by different nations and cities at different times.  Some Eastern Churches still use the Julian calendar.  So, all times before Oct 15, 1582 are pre-Gregorian times.  However, for years 1582 or greater, some nations, cities, and Churches use Gregorian, and some do not.  To determine whether a given year is Gregorian for a given place, consult the chart below:

PLACE              FIRST GREGORIAN DATE
                   (year, mo, da)
Albania             1912, 12, ??
Augsburg            1583, 02, 24
Baden               1583, 11, 27
Bavaria             1583, 10, 16
Belgium (Did not exist. Use Netherlands.)
Bohemia             1584, 01, 17
British Colonies    1752, 09, 14
Bulgaria            1915, 11, 14 or 1916-04-14?
Carynthia           1583, 12, 25
China               1949
Cologne             1583, 11, 14 or 1583-11-13?
Czech Republic:
  (Did not exist. Use Bohemia and Moravia.)
Denmark             1700, 03, 01
Estonia             1918, 02, 14
Egypt               1875, ??, ??
Faeror              1700, 11, 28 or 1700-11-27?
France              1582, 12, 20
  Alsace            1682, 02, 16
  Lorraine          1760, 02, 28
  Strasbourg        1682, 02, 16
  Strasbrg. diocese 1583, 11, 27
Finland (Did not exist. Use Sweden.)
Germany Protestant  1700, 03, 01
Great Britain       1752, 09, 14
  Ireland           1752, 09, 14
  Scotland          1752, 09, 14
  Wales             1752, 09, 14
Greece              1924, 03, 23
Hildesheim          1631, 03, 26
Holland             1583, 01, 01 or 1583-1-12?
Hungary             1587, 11, 01
Iceland             1700, 11, 28
Italy               1582, 10, 15
Japan               1873, 01, 01
Julich              1583, 11, 13
Latvia              1918, 02, 15
Liege               1583, 02, 21
Lithuania           1915, ??, ??
Luxembourg          1582, 12, 25
Mainz               1583, 11, 22
Minden              1668, 02, 12
Moravia             1584, 01, 17
Netherlands
  Drente            1701, 05, 12 or 1701-01-12?
  Friesland         1701, 01, 12 or 1701-01-13?
  Holland           1583, 01, 12
  Gelderland        1700, 07, 12
  Limburg & southern provinces
    (now Belgium)   1582, 12, 31
  Staten Generaal   1582, 12, 25
  Utrecht           1700, 12, 12
  Zeeland, Brabrant 1582, 12, 25
Norway (Did not exist. Use Denmark.)
Paderborn           1585, 06, 27
Portugal            1582, 10, 15
Poland              1582, 10, 15
Prussia             1610, 09, 02
Romania             1919, 04, 14 or 1924-10-14?
Russia              1918, 02, 14
Silesia             1584, 01, 23
Spain               1582, 10, 15
Treves              1583, 10, 15
Sweden              1753, 03, 01
Switzerland (local variations)
  Catholic cantons  1583, 1584 or 1597
  Protestant cantn. 1701, 01, 12
  Lucerne           1584, 01, 22
  Zurich            1701, 01, 12
Transylvania        1590, 12, 25
Turkey              1927, 01, 01
Tyrolia             1583, 10, 16
U.S.A. (Did not exist as such.)
  Alaska            1867, 10, ??
  Eastern seaboard: Use Great Britain, 1752
  Mississippi valley: Use France, 1582
  NW Terr: Use Great Britain, 1752
  Southern parts: Use Spain, 1582
Westphalia          1584, 07, 12
Wurzburg            1583, 11, 15
Yugoslavia          1919, ??, ??

I compiled the above chart from:

Christian.pm by Eugene van der Pijll, and
The Christian Calendar by Nikos Drakos (University of Leeds), Marcus Hennecke, Ross Moore, Herb Swan, Jens Lippmann, Marek Rouchal, and Martin Wilck.