Good Friday is the Friday before Easter, which is calculated differently in Eastern Christianity
and Western Christianity . 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. See Easter Dating Method (Astronomical Society of South Australia).
Because Easter in Western Christianity can fall between March 22 and April 25 on the Gregorian calendar,
Good Friday can fall between March 20 and April 23, inclusive. In Eastern Christianity, Easter can fall
between March 22 and April 25 on Julian Calendar (thus between April 4 and May 8 in terms of the Gregorian
calendar, during the period 1900 and 2099), so Good Friday can fall between March 20 and April 23, inclusive
(or between April 2 and May 6 in terms of the Gregorian calendar).