There is an experimental WWW page which does conversions between the Julian
and Gregorian calendars, calculates the day of the week and the date of Easter
for the year in question.

This is at  http://wwwcn.cern.ch/%7Emcnab/n/PerCalForm.html  and is part of
a collection of material about Sir Isaac Newton. There is also a link to a list
of calendrical resources on the web on that page. So far it has been checked
using some of the dates in Pepys' Diary and various others from biographies of
Newton.

If you really are intending to do this manually then many books on astronomy
have
an introductory chapter about calendars which would be useful. The difference
in days between the two calendars changes at the end of each century since
1700, 1800 and 1900 were all Julian but not Gregorian leap years.

Hope this helps,

 Andrew McNab

PS. Does anyone on the list have access to a late C17th English Book of Common
    Prayer (I think it would be the ?1662? edition) If the method of
    calculating Easter is reasonably short then I would really appreciate a
copy
    of it so I can compare the results with the Gregorian date of Easter.
    Using the Gregorian Easter and then converting to the Julian date seemed to
    correctly predict the date of Easter in Pepys Diary but I want to check
    properly. (I'm in Geneva and 1662 Books of Common Prayer are not thick on
the
    ground...)