Calculating the date of Thanksgiving
Having a birthday on the 26th of November means that, every so often, it coincides with Thanksgiving. Last night, lying awake in the dark and listening to the rain beat down, I decided to finally address the question of exactly when I could expect this to occur in the future, rather than letting it sneak up on me over the ensuing years.
Here’s the way I came up with, which assumes John Horton Conway’s Doomsday algorithm:
- Find Doomsday for a given year.
- Let x equal the number of days between Thursday and the following Doomsday. This can be computed as (7 + Doomsday - 4) mod 7, 4 being the day of the week corresponding to Thursday.
- The Doomsday following or on Thanksgiving is November 28th, so the date of Thanksgiving is equal to 28 - x.
For example, this year Doomsday falls on Friday = 5. So Thanksgiving is on the (28 - (7 + 5 - 4) mod 7)th = 27th.
Filtering dates of Thanksgiving for this century for those that occur on my birthday yields the years 2009, 2015, 2020, 2026, 2037, 2043, 2048, 2054, 2065, 2071, 2076, 2082, and 2093. The last time my birthday was on Thanksgiving was 1998.
