My understanding was that Good Friday is the first Friday after the first full moon after the equinox. I suppose this sounds pretty pagan, but I think it is much more interesting than always being the 21st day of the third month which is just as 'unreligious'. For Good Friday to be on the equinox we'd need the equinox to happen on the same day as the full moon and that day should be a Friday.
What I have discovered however is:
That it is actually the date of Easter Sunday that is set. It is the first Sunday after the first full moon after the equinox. Here.
Also the equinox this year actually falls on the 20th of March (at about 5am GMT). Last year it was the 21st, but it is apparently quite often the 20th! See here.
The full moon will be on 21st March at 6.40pm GMT.
The last time Easter Sunday was on the 23 March was in 1913 and will be again in 2160. The earliest Easter Sunday can be is 22 March (1818 and 2285).
Interesting! And cool to have equinox, full moon and Easter Sunday almost a day after each other.
The picture is John van der Laar's.