Why doesn GOOD FRIDAY fall on a fixed date every year?
According to Wikipedia : Good Friday is the Friday before Easter, which is calculated differently in Eastern Christianity and Western Christianity (see Computus for details). Easter falls on the first Sunday following the Paschal Full Moon, the full moon on or after 21 March, taken to be the date of the vernal equinox. The Western calculation uses the Gregorian calendar, while the Eastern calculation uses the Julian calendar, whose 21 March now corresponds to the Gregorian calendar’s 3 April. The calculations for identifying the date of the full moon also differ.