i'm using the Astro_MoonPhase class to calculate the full moon after the equinox and from there it's easy enough to use native php functions to count the days backwards.

unfortunately i haven't yet found a way of automatically calculating the date of the equinox (that i can understand), which would mean the calculations could be astronomically correct depending on your location/timezone. so for the moment i just have a manually entered list of the equinox dates and it's based on brisbane, qld, australia.

2025
      Vernal equinox is on September 23rd at 04:19
      Full moon is on October 7th (Tuesday)
      Easter is on October 12th (Sunday)
      Ash Wednesday is on August 27th
      Shrove Tuesday is on August 26th 2025

2026
      Vernal equinox is on September 23rd at 10:05
      Full moon is on September 26th (Saturday)
      Easter is on September 27th (Sunday)
      Ash Wednesday is on August 12th
      Shrove Tuesday is on August 11th 2026

2027
      Vernal equinox is on September 23rd at 16:01
      Full moon is on October 15th (Friday)
      Easter is on October 17th (Sunday)
      Ash Wednesday is on September 1st
      Shrove Tuesday is on August 31st 2027

2028
      Vernal equinox is on September 22nd at 21:45
      Full moon is on October 3rd (Tuesday)
      Easter is on October 8th (Sunday)
      Ash Wednesday is on August 23rd
      Shrove Tuesday is on August 22nd 2028

2029
      Vernal equinox is on September 23rd at 03:38
      Full moon is on October 22nd (Monday)
      Easter is on October 28th (Sunday)
      Ash Wednesday is on September 12th
      Shrove Tuesday is on September 11th 2029

2030
      Vernal equinox is on September 23rd at 09:26
      Full moon is on October 11th (Friday)
      Easter is on October 13th (Sunday)
      Ash Wednesday is on August 28th
      Shrove Tuesday is on August 27th 2030

2031
      Vernal equinox is on September 23rd at 15:15
      Full moon is on September 30th (Tuesday)
      Easter is on October 5th (Sunday)
      Ash Wednesday is on August 20th
      Shrove Tuesday is on August 19th 2031

2032
      Vernal equinox is on September 22nd at 21:10
      Full moon is on October 18th (Monday)
      Easter is on October 24th (Sunday)
      Ash Wednesday is on September 8th
      Shrove Tuesday is on September 7th 2032

2033
      Vernal equinox is on September 23rd at 02:51
      Full moon is on October 8th (Saturday)
      Easter is on October 9th (Sunday)
      Ash Wednesday is on August 24th
      Shrove Tuesday is on August 23rd 2033

2034
      Vernal equinox is on September 23rd at 08:39
      Full moon is on September 28th (Thursday)
      Easter is on October 1st (Sunday)
      Ash Wednesday is on August 16th
      Shrove Tuesday is on August 15th 2034

2035
      Vernal equinox is on September 23rd at 14:39
      Full moon is on October 17th (Wednesday)
      Easter is on October 21st (Sunday)
      Ash Wednesday is on September 5th
      Shrove Tuesday is on September 4th 2035

2036
      Vernal equinox is on September 22nd at 20:23
      Full moon is on October 5th (Sunday)
      Easter is on October 12th (Sunday)
      Ash Wednesday is on August 27th
      Shrove Tuesday is on August 26th 2036

2037
      Vernal equinox is on September 23rd at 02:13
      Full moon is on September 24th (Thursday)
      Easter is on September 27th (Sunday)
      Ash Wednesday is on August 12th
      Shrove Tuesday is on August 11th 2037

2038
      Vernal equinox is on September 23rd at 08:02
      Full moon is on October 13th (Wednesday)
      Easter is on October 17th (Sunday)
      Ash Wednesday is on September 1st
      Shrove Tuesday is on August 31st 2038

2039
      Vernal equinox is on September 23rd at 13:49
      Full moon is on October 2nd (Sunday)
      Easter is on October 9th (Sunday)
      Ash Wednesday is on August 24th
      Shrove Tuesday is on August 23rd 2039

2040
      Vernal equinox is on September 22nd at 19:44
      Full moon is on October 20th (Saturday)
      Easter is on October 21st (Sunday)
      Ash Wednesday is on September 5th
      Shrove Tuesday is on September 4th 2040

2041
      Vernal equinox is on September 23rd at 01:26
      Full moon is on October 9th (Wednesday)
      Easter is on October 13th (Sunday)
      Ash Wednesday is on August 28th
      Shrove Tuesday is on August 27th 2041

2042
      Vernal equinox is on September 23rd at 07:11
      Full moon is on September 29th (Monday)
      Easter is on October 5th (Sunday)
      Ash Wednesday is on August 20th
      Shrove Tuesday is on August 19th 2042

2043
      Vernal equinox is on September 23rd at 13:06
      Full moon is on October 18th (Sunday)
      Easter is on October 25th (Sunday)
      Ash Wednesday is on September 9th
      Shrove Tuesday is on September 8th 2043

2044
      Vernal equinox is on September 22nd at 18:47
      Full moon is on October 7th (Friday)
      Easter is on October 9th (Sunday)
      Ash Wednesday is on August 24th
      Shrove Tuesday is on August 23rd 2044

2045
      Vernal equinox is on September 23rd at 00:32
      Full moon is on September 26th (Tuesday)
      Easter is on October 1st (Sunday)
      Ash Wednesday is on August 16th
      Shrove Tuesday is on August 15th 2045

2046
      Vernal equinox is on September 23rd at 06:21
      Full moon is on October 14th (Sunday)
      Easter is on October 21st (Sunday)
      Ash Wednesday is on September 5th
      Shrove Tuesday is on September 4th 2046

2047
      Vernal equinox is on September 23rd at 12:08
      Full moon is on October 3rd (Thursday)
      Easter is on October 6th (Sunday)
      Ash Wednesday is on August 21st
      Shrove Tuesday is on August 20th 2047

2048
      Vernal equinox is on September 22nd at 18:00
      Full moon is on October 21st (Wednesday)
      Easter is on October 25th (Sunday)
      Ash Wednesday is on September 9th
      Shrove Tuesday is on September 8th 2048

2049
      Vernal equinox is on September 22nd at 23:42
      Full moon is on October 11th (Monday)
      Easter is on October 17th (Sunday)
      Ash Wednesday is on September 1st
      Shrove Tuesday is on August 31st 2049

i'm attempting to create a script for easily calculating the southern hemisphere equivalent of shrove tuesday.

the date of ash wednesday is determined by counting backwards from the date of easter. shrove tuesday is the day before ash wedesday. there are many programs and web scripts that will easily let you determine the date of easter, but the problem with these is that they are all based upon northern hemisphere traditions. since historical evidence strongly suggests that easter came to be named and scheduled as it is due to already existing pagan customs celebrating the coming spring during Eostur-monath (april), it makes sense to adjust these dates to account for the differing seasons in the southern hemisphere.

i'm keeping my calculations simple and using the basic rule given by bede for the date of easter: The Sunday following the full Moon which falls on or after the equinox will give the lawful Easter. the churchy folks in the northern hemisphere actually make it a lot more complex than that because they decided that the vernal (spring) equinox should be considered to always be on march 21st (when it can actually fall on the 20th, 21st or 22nd of march) and their "paschal full moon" does not even necessarily fall on the day of a true full moon. you can read all about that on wikipedia, if you're so inclined.

so my 'formula' for working this out goes along these lines:

start at: spring equinox
(southern hemisphere seasons)

determine: full moon
that occurs either on or after the date of the equinox

determine: sunday
after the full moon
this is our southern hemisphere "easter"

count back: 46 days
this is forty days if you exclude the sundays.. which you do
this is the date of ash wednesday, the first day of lent

count back: 1 day
this is shrove tuesday. let's eat pancakes!

© 2011 jade e. leth
yes, i am aware of the irony of calculating southern hemisphere equivalents
for "stolen" pagan festivals turned into christian celebrations. that's kind of why it's fun.
also, it legitimises the eating of pancakes, and that can never be a bad thing.