Author: Manuel Sarmiento

Here is another function that calculates the Good Thursday and any other related 
date for eny year. The algorithm provided here is straightforward in the sense that 
it calculates the Good Thursday (which is the jewish passover) as the thursday 
ocurring in the same week as the first spring full moon. Obviously the function can 
be easily adapted to calculate any full moon down to the second.

Answer:

1   function good_thursday(year: integer): tdatetime;
2   const
3     full_moon: tdatetime = 34804.33889; {15/4/95 8:08}
4     sunday: tdatetime = 1;
5     sinodic_month: tdatetime = 29.53058912;
6   
7   var
8     equinoccio: tdatetime;
9     lunar_months: double;
10    full_moon, weeks: double;
11  
12  begin
13    if year < 100 then
14      if year year := year + 2000
15    else
16      year := year + 1900;
17    equinoccio := encodedate(year, 3, 21);
18    lunar_months := 10000 - Int(10000 - (equinoccio - full_moon) / sinodic_month);
19    full_moon := full_moon + sinodic_month * lunar_months;
20    weeks := 10000 - Int(10000 - (full_moon - sunday) / 7);
21    good_thursday := sunday + 7 * weeks - 3;
22  end;