Using the Google ngrams corpus, xkcd sized the days of the year based on usage volume. Lots of firsts of the month and September 11th.
I’m a fan of XKCD’s “sketchy” graphics (and his interesting data collection using systematic searches), but in this case particularly I think it would be better as clean typeface. I want to see some of the subtle distinctions, but I have trouble making comparisons because of the sketchiness of the font.
I wonder what this would look like in other languages. Certainly April 15th (for example) gets a boost because it is tax day in the U.S, but I imagine that the 1st is referenced relatively evenly across languages,