Calculating Easter

It's Easter weekend, and as promised we're going to write a function to calculate when Easter occurs for a given year. Easter is a strange holiday in that it jumps around the calendar between March and April. This is because it is tied to the lunar cycle. Easter occurs on the first Sunday after the Paschal full moon.

ASP

1. function dateEaster(someYear)
2.     Dim goldenNumber
3.     Dim solarCorrection
4.     Dim lunarCorrection
5.     Dim paschalFullMoon
6.     Dim dominicalNumber
7.     Dim difference
8.     Dim dayEaster
10.     goldenNumber = (someYear Mod 19) + 1
12.     if someYear <= 1752 then
13.         ' Julian calendar
14.         dominicalNumber = (someYear + (someYear / 4) + 5) Mod 7
15.         paschalFullMoon = (3 - (11 * goldenNumber) - 7) Mod 30
16.     else
17.         ' Gregorian calendar
18.         dominicalNumber = (someYear + (someYear / 4) - (someYear / 100) + (someYear / 400)) Mod 7
19.         solarCorrection = (someYear - 1600) / 100 - (someYear - 1600) / 400
20.         lunarCorrection = (((someYear - 1400) / 100) * 8) / 25
21.         paschalFullMoon = (3 - 11 * goldenNumber + solarCorrection - lunarCorrection) Mod 30
22.     end if
24.     do until dominicalNumber > 0
25.         dominicalNumber = dominicalNumber + 7
26.     loop
28.     do until paschalFullMoon > 0
29.         paschalFullMoon = paschalFullMoon + 30
30.     loop
32.     if paschalFullMoon = 29 or (paschalFullMoon = 28 and goldenNumber > 11) then
33.         paschalFullMoon = paschalFullMoon - 1
34.     end if
36.     difference = (4 - paschalFullMoon - dominicalNumber) Mod 7
38.     if difference < 0 then
39.         difference = difference + 7
40.     end if
42.     dayEaster = paschalFullMoon + difference + 1
44.     if dayEaster < 11 then
45.         ' Easter occurs in March.
46.         dateEaster = DateSerial(someYear, 3, dayEaster + 21)
47.     else
48.         ' Easter occurs in April.
49.         dateEaster = DateSerial(someYear, 4, dayEaster - 10)
50.     end if
51. end function

PHP

1. function dateEaster($someYear)
2. {
3.     $goldenNumber = fmod($someYear, 19) + 1;
5.     if ($someYear <= 1752)
6.     {
7.         // Julian calendar
8.         $dominicalNumber = fmod($someYear + ($someYear / 4) + 5, 7);
9.         $paschalFullMoon = fmod(3 - (11 * $goldenNumber) - 7, 30);
10.     }
11.     else
12.     {
13.         // Gregorian calendar
14.         $dominicalNumber = fmod($someYear + ($someYear / 4) - ($someYear / 100) + ($someYear / 400), 7);
15.         $solarCorrection = ($someYear - 1600) / 100 - ($someYear - 1600) / 400;
16.         $lunarCorrection = ((($someYear - 1400) / 100) * 8) / 25;
17.         $paschalFullMoon = fmod(3 - 11 * $goldenNumber + $solarCorrection - $lunarCorrection, 30);
18.     }
20.     while ($dominicalNumber < 0)
21.     {
22.         $dominicalNumber += 7;
23.     }
25.     while ($paschalFullMoon < 0)
26.     {
27.         $paschalFullMoon += 30;
28.     }
30.     if ($paschalFullMoon == 29 || ($paschalFullMoon == 28 && $goldenNumber > 11))
31.     {
32.         $paschalFullMoon--;
33.     }
35.     $difference = fmod(4 - $paschalFullMoon - $dominicalNumber, 7);
37.     if ($difference < 0)
38.     {
39.         $difference += 7;
40.     }
42.     $dayEaster = $paschalFullMoon + $difference + 1;
44.     if ($dayEaster < 11)
45.     {
46.         // Easter occurs in March.
47.         $dateEaster = mktime(0, 0, 0, 3, $dayEaster + 21, $someYear);
48.     }
49.     else
50.     {
51.         // Easter occurs in April.
52.         $dateEaster = mktime(0, 0, 0, 4, $dayEaster - 10, $someYear);
53.     }
55.     return $dateEaster;
56. }

We can calculate the other ecclesiastical holidays by offsetting the date of Easter with the number of days between Easter and the holiday we're looking for.

ASP

1. function dateGoodFriday(someYear)
2.     dateGoodFriday = DateAdd("d", -2, dateEaster(someYear))
3. end function
5. function datePalmSunday(someYear)
6.     datePalmSunday = DateAdd("d", -7, dateEaster(someYear))
7. end function
9. function dateAshWednesday(someYear)
10.     dateAshWednesday = DateAdd("d", -46, dateEaster(someYear))
11. end function
13. function dateAscensionDay(someYear)
14.     dateAscensionDay = DateAdd("d", 39, dateEaster(someYear))
15. end function
17. function datePentecost(someYear)
18.     datePentecost = DateAdd("d", 49, dateEaster(someYear))
19. end function

PHP

1. function dateGoodFriday($someYear)
2. {
3.     $easter = getDate(dateEaster($someYear));
4.     return mktime(0, 0, 0, $easter[mon], $easter[mday] - 2, $easter[year]);
5. }
7. function datePalmSunday($someYear)
8. {
9.     $easter = getDate(dateEaster($someYear));
10.     return mktime(0, 0, 0, $easter[mon], $easter[mday] - 7, $easter[year]);
11. }
13. function dateAshWednesday($someYear)
14. {
15.     $easter = getDate(dateEaster($someYear));
16.     return mktime(0, 0, 0, $easter[mon], $easter[mday] - 46, $easter[year]);
17. }
19. function dateAscensionDay($someYear)
20. {
21.     $easter = getDate(dateEaster($someYear));
22.     return mktime(0, 0, 0, $easter[mon], $easter[mday] + 39, $easter[year]);
23. }
25. function datePentecost($someYear)
26. {
27.     $easter = getDate(dateEaster($someYear));
28.     return mktime(0, 0, 0, $easter[mon], $easter[mday] + 49, $easter[year]);
29. }