You know the saying: "you learn something new every day"
Well it seems more likely that it is Monday, Wednesday or Friday. The reason being that XKCD almost always has some deeper meaning or higher maths that involves googling and reading of wikipedia to fully understand.
Even today's - I did not know what a Shibboleth was and now know that Shibboleet is a Shebboleth and a play on words as well. See the wikipedia on it to fully understand.
I should have guessed it was not just a random made up word, but something "clever" too.