I’ve always had a nerdy interest in all things calendar and timekeeping, so I made an effort these days to memorise the “doomsday algorithm”, which is a method developed by mathematician John Conway to compute the day of the week for a given day mentally. It seems to exist in various versions (because subsequent writers have published improvements upon Conway’s initial method), but I think I’m now reasonably familiar with one of them. Gets me the day of the week for any date (since 1600) in about 20-30 seconds, which is of course a lot but I’m confident that with practice I can reduce that; for the current year it’s much faster (because a major part of the algorithm is to compute a certain value for the given year, which you might as well just memorise for the current one).
Of course it’s completely useless, and for things that matter I’d still look the day up properly; but it’s a cool skill to have, not very difficult to learn, and maybe it makes for a neat party trick.