The LUNAR calendar makes it simple. The NEW MOON starts a New Month the first being the first sighting of a sliver of light on the crescent moon. The 14th is always the FULL MOON, 14 days later, the cycle repeats.
Now for the bible. The last supper was on a full moon (try running around the desert in the dark), Nisan 14. So Easter doesn't shift around arbitrarily, but always falls on the same date in the Jewish calendar. How simple.
If I understand this then, Christians use Sundays (The better to work like mad the other six days) and so if Nisan 14 doesn't land on a Friday, it gets moved to the next available Friday and so Easter moves to the following Sunday.
Nisan 14, After Sundown
Jerusalem is shrouded in the soft light of dusk as the full moon rises over the Mount of Olives. In a large furnished room, Jesus and the 12 are reclining at a prepared table. "I have greatly desired to eat this passover with you before I suffer," he says. (Luke 22:14, 15)
Reliving Jesus' Last Days on Earth - Jehovah's Witnesses Official Web Site
Happily the Islamic or Hijri calendar is also lunar and the most common on earth; but since it requires that the crescent moon be observed locally to start a month, the universal version is considered but an approximation of nature's correct and observable clock.
More lunar calendars
The Hellenic calendars, the Hebrew Lunisolar calendar and the Islamic Lunar calendar started the month with the first appearance of the thin crescent of the new moon.However, the motion of the Moon in its orbit is very complicated and its period is not constant. The date and time of this actual observation depends on the exact geographical longitude as well as latitude, atmospheric conditions, the visual acuity of the observers, etc. Therefore the beginning and lengths of months in these calendars can not be accurately predicted.While some like the Karaites Jews still rely on actual moon observations, most people use the Gregorian solar calendar.[edit]Julian and Gregorian calendarsThe Gregorian calendar, like the Julian calendar before it, has twelve months:Month - Wikipedia, the free encyclopedia
