"The civil calendar in use around the world is a solar calendar based
on the progression through the seasons as the Earth revolves around
the Sun. A lunar calendar bases each month on a full cycle of the
Moon's phases (called a lunation or synodic month). Lunar calendars
usually start each month with a New Moon or the first visible crescent
moon after New Moon. The solar year does not contain an integral
number of days or an integral number of lunar months..."
Another good run down of the calendars is on :
We currently use (those of us in the Civil sector of life), the
Gregorian Calendar. Leap year rules for the Gregorian Calendar are
"every year that is divisible by 4 is a leap year, except for years
which are exactly divisible by 100. An example would be that 1900 is
not a leap year, where 2000 is a leap year.
The dates for Easter are given by a set of ecclesiastical rules and
First off, Easter is to be on a Sunday (this of course suggests it
should be the same Sunday world wide).
The first tables to fix the Easter day to the Gregorian Calendar was
created by Abbot of Scythia Dionysis Exiguus. Most of the time you
hear the statement that "Easter Day is the first Sunday after the full
moon that occurs next after the vernal equinox", which is "sort of"
true. Really, the full moon they are talking about is not the
astronomical Full Moon, but the ecclesiastical full moon (determined
from tables). More or less it is the same thing, but if there is a
difference, then the ecclesiastical tables win out, no matter what the
moon in the sky thinks it is doing.
The vernal equinox is fixed as March 21, so Easter can never occur
before March 22, or after April 25. The Gregorian dates for the
ecclesiastical full moon come from the Gregorian tables. Many eastern
orthodox Christian churches still use the older tables, which are
based on the Julian calendar.
for more about this see "The Date of Easter"
( http://aa.usno.navy.mil/faq/docs/easter.html )
The common belief is that the Last Supper took place on Passover,
which is the fourteenth day of the first month of the Jewish calendar.
The Jewish calendar was/is lunar based so this month always started
with a full moon. Fourteen days after that day is always on or very
close to the vernal equinox (first day of spring in common tongue).
Easter is recorded as being on Sunday, seventeen days after the
paschal moon. This being the case, an exact anniversary (falling on
Sunday) is only possible about once every 19 years.
Eastern and Western churches had quite a disagreement over this. The
Eastern churches believed that the "Sunday" part didn't mater, and
Easter should be celebrated on the 14th day, and the Westerns
believing it should be on the Sunday after the 14th day of the moon.
For further information on Epact and the setting of Easter by the
Church, see the Epact entry in the Catholic Encyclopedia
links of interest
Six Major Calendars
Gregorian Lunar Calendar
Also see other calendars on this site using the home page and this link
calendar, Gregorian, solar, lunar, and dates of Easter and Passover
The Liturgical Year
Why does Easter's Date Wander?
The Date of Easter