to calculate holidays

How can I calculate Easter, or other holidays

Easter is probably the most difficult of all since it is not a 'fixed' date.
If I am not mistaken, the date it falls on is determined by the phases
of the moon and I am not sure what the specific algorithm is. Many
of the other holidays involve something like 'the second Monday of
the month'. That is, in the US where we have this fondness for
three-day weekends. Sorry I could not be more helpful but you have
made me curious. Time to do some searching.

Well, I found several algorithms. You should check them out and
decide for yourself which to use. I found them by going to
altavista and using this search phrase : Easter +algorithm.

Use Gauss Easter Formula
I was using that one in a previous job I had
It works fine for the werstern calendar, but will not do the job for Orthodox churches in eastern Europe

I you want to check a date and tell if is a holiday or not then I do this like:

make a database table with listing of all holidays in certain period.
if you want to check a date to holiday simply run an SQL matching the date. If it is found you know it is holiday!

hope this helps!