Like many people born in late November, my birthday occasionally falls on Thanksgiving Day – on average it happens once every seven years, but the pattern is complicated by the subtleties of the Gregorian calendar. Out of idle curiosity I went looking online to find out which years I’d be having turkey and cranberries on my birthday, but the search engines came up empty. (I was sort of hoping Wolfram Alpha could pull an answer out of its knowledge engine, but to no avail.)

So, for posterity’s sake: Thanksgiving is always on the fourth Thursday in November, which means the holiday always falls between November 22nd and 28th. The simple formula for calculating the date of Thanksgiving is to take last year’s date and subtract a day, and then add seven days if you’ve ended up on the 21st… so if Thanksgiving 2009 was on the 26th (which it was), then Thanksgiving 2010 is on the 25th, and in 2011 it’ll be on the 24th.

But then things start getting complicated, because 2012 is a leap year. Thanksgiving 2012 is on the 22nd, not the 23rd – that extra day in February pushes us back two days instead of just one. And then, in 2013, we wrap around: One day earlier than the 22nd would be the 21st, but now we’re looking at the third Thursday of the month, so we add seven days to get Thanksgiving on the 28th.

All this adds up to a pattern that takes 2800 years to go through a full cycle, thanks to our calendar’s quirky rules for determining whether it’s a leap year or not. In the short term (that is, from 1901 to 2099) there’s a leap year every four years, and your birthday will be on Thanksgiving four times out of every 28: If your birthday falls on Thanksgiving in a given year, then it falls on Thanksgiving again exactly 28 years later, and three other times in between at intervals of six years, five years, six years, and then eleven years.

And so, to answer my own question (and with the help of a handy spreadsheet), here’s a list of the years between now and 2100 when Thanksgiving falls on a particular date. Plan your birthdays accordingly.

November 22nd 2012, 2018, 2029, 2035, 2040, 2046, 2057, 2063, 2068, 2074, 2085, 2091, 2096 November 23rd 2017, 2023, 2028, 2034, 2045, 2051, 2056, 2062, 2073, 2079, 2084, 2090 November 24th 2016, 2022, 2033, 2039, 2044, 2050, 2061, 2067, 2072, 2078, 2089, 2095 November 25th 2021, 2027, 2032, 2038, 2049, 2055, 2060, 2066, 2077, 2083, 2088, 2094, 2100 November 26th 2015, 2020, 2026, 2037, 2043, 2048, 2054, 2065, 2071, 2076, 2082, 2093, 2099 November 27th 2014, 2025, 2031, 2036, 2042, 2053, 2059, 2064, 2070, 2081, 2087, 2092, 2098 November 28th 2013, 2019, 2024, 2030, 2041, 2047, 2052, 2058, 2069, 2075, 2080, 2086, 2097

This post is probably a sign that I have too much free time, but it’s one of those nagging little questions that I’d wondered about occasionally for years and never got around to finding out… and then discovered that the vast storehouse of information on the Web didn’t actually have the answer yet. So here it is.