I never learned the "30 days has September..." mnemonic as a kid. In my high school German class, we were taught it freshman year as something cool we could rattle off and feel like we spoke German. Thus, the only way I can remember if a month has 30 or 31 days (like say, when it's two in the morning and I'm mapping out my homework to-do list for the week), is to say it in German to myself. Like Dwight Schrute. I will now do it for you, and please German grammaticians (and I know there's a lot of you), don't judge the spelling.
Dreissig tage hat September, April, Juni und November. Alle die noch ubrig blieben haben ein un dreissig ausser Februar.
I probably have to do this, on average, like three or four times a month.Pretty useful! Not nearly as useful as knowing Spanish, for instance, but what can you do?