The riddle
When Does Christmas Come Before Thanksgiving?
Letters decide, not dates
On the calendar, Thanksgiving comes first. The United States sets it on the fourth Thursday of November and Christmas Day on December 25, and federal law lists both as holidays (5 U.S.C. 6103). The riddle never asks about dates, though. It asks where one word comes before the other, and in dictionary order, words follow the alphabet. Christmas begins with C, the third letter, and Thanksgiving with T, the twentieth, so Christmas is listed first. Sixteen letters lie between C and T. Length plays no part: Thanksgiving has twelve letters and Christmas nine, but the first letter decides.
When the first letters match
Two words that start with the same letter are sorted by the second letter, and ties there are settled by the third, and so on. Chill and Christmas both open with c and h, so the third letters decide: i comes before r, and chill is listed first. Dictionary makers apply that rule to every entry, which is why a word can be found knowing nothing about it except its spelling.
Same trick, different words
Any two words can be tested the same way. Pick a pair, such as autumn and winter, and ask when the first comes before the second. The reply is the same each time: when the first letters are in alphabetical order, which for that pair they are. Swap the pair around and the trick fails, because winter sits late in the alphabet. For another riddle built on the same kind of trick, see the riddle about today and yesterday.