July 2007 has a Friday the 13th. Any month that starts on a Sunday will have a Friday the 13th.
There are only 14 unique yearly calendars in the current (Gregorian) system: 7 for regular years and 7 for leap years; each one starting on a different day of the week.
What are some other obvious things that you had to think about before they became obvious?
Once lived in a building that had two elevator banks. At first, always confused about where the floor buttons would be when I got in an elevator. Finally dawned on me that you turn towards the other elevator.
Just in case the Friday the 13th issue has piqued your interest, the Wikipedia article Friday the 13th has some other amusing facts and legends, some obvious and some not so much.
One noteworthy detail from the article is that the 13th of the month is more likley to occur on Friday than on any other day!
The elevators in my office are the other way around. I was always turning to the wrong side when I finally got it nailed down as “Buttons are on the outside.”
And which side does the middle of three elevators have its buttons?
Not at all obvious, but on the theme of calendar oddities:
May has the unique property that both its first day and its last day are the only time in the year that the month starts or ends on that day of the week. For example, this year May 1st is the only “Tuesday 1st”, and May 31st is the only Thursday that is the last day of the month. It works in leap years too.
I only know this because I once tried to work it up into a brainteaser, but as you can see the wording gets a bit complicated 
I note from that article that Rossini died on a Friday the 13th. It was the same year that he had just celebrated his 18th birthday!
nemmind
Except in leap years, any given date in March will fall on the same day of the week as it did in February. If March 14th was on a Wednesday, so was February 14th.
In high school I used to experiment with different handwriting styles – yeah, yeah, I was a real hoot at parties. For example, I turned in all of my homework one semester written in backhand (wherein the letters slant thuswise: \\\). So on a whim I started taking notes one day in a Roman inscription type of writing, in all caps and substituting V and I in place of u or j. At some point I looked back at something I’d just written, and was completely stunned. When written in Roman style letters, the name Juan comes out as IVAN.
I wasn’t so much embarrassed that I’d never realized it before, as with how much it took me by surprise.
I’ve been unable to find an official-sounding etymology to support the idea that Europe comes from the same root as Ural (mountains and river) but it seems obvious to me that they have something in common beyond their sound. Maybe I would find that confirmation if I kept looking…
Another questionable etymology, supported at least in this web article at Answers.com on Red Cedar, is that Baton Rouge (English = Red Stick) may be named for the red wood in what’s referred to in this part of the USA as Cedar or Red Cedar, but which is really a variety of Juniper. Another neat connection is that the word Gin (the alcoholic drink used in martinis and such) is derived from the word Juniper in a roundabout way.
The Baton Rouge (Red Stick) name may have a more prosaic origin from a bloodstained pole marking a boundary between Indian tribes.
As with most etymologies, a fair amount of guesswork is probably involved. Or bullshit.