This year February 6 is, obviously, a Sunday. Everybody knows about the days of the week. In the US, Sunday is the first day of the week — all you have to do to know that is glance at a wall calendar. Elsewhere in the world, though, it’s different. In the Middle East (most of it, anyway), the first day of the week is Saturday. In Europe, the first day is Monday. And the “official” first day of the week is Monday, too — it says so in the international ISO standard, number 8601.

Regardless of which day you pick to start counting, though, everybody in the world seems to agree that there is such a thing as a week, that it’s seven days long, that each day has a name (although the names might differ), and that at least one of the days isn’t for work. If you’re lucky, you have two of them off each week.

If you think about it, this is an astonishing example of international cross-cultural understanding, agreement, and cooperation. People can’t agree on a common language, after all. Linear distances are sometimes inches, sometimes meters, and if you really want some variety, distances can also be in leagues, nautical miles, rods, furlongs, or chains. And those are the easy, basic things. Think about money — we can’t agree on a currency any more than we can agree on an economic system. Politics, food, music, what to wear, what particular hand gestures mean, which is better, dogs or cats…it’s chaotic. And that’s not even considering religion. People, as a rule, can’t seem to agree about much of anything.

Except weeks. The idea that a week is seven days hasn’t been universal, but it’s old. It goes back at least as far as 2100 BCE. We know that because there was a leader named Gudea, the  king of the ancient Sumerian city of Lagash who built a temple with seven rooms, each apparently dedicated to some astrological figure that represented a day. Nobody knows how much further back the seven-day cycle goes, but just keep in mind that in 2100 BCE, the Sumer civilization had already been around for two or three thousand years. The seven-day week might be very old indeed.

There have been some other ideas now and then, of course. In ancient Rome, for a while, they had an eight-day week. In ancient Chinese and Egyptian cultures, there was a ten-day week — and that idea was briefly resurrected in the French Republic after the 1793 revolution. They gave up on it after just 12 years, though. In the Americas, the Aztecs and Mayas had weeks too, but they were 13 days long.

However, I think I’ve found the champion of weekish complexity. It just might be Pawukon calendar from Bali. That calendar has weeks of different lengths, from one to ten days. The weeks all start at the same time, on the first day of the year, which is 210 days long. The weeks keep running concurrently for the whole year, and conclude at year’s end, just in time for the whole thing to start over. Except that the 4-, 8-, and 9-day weeks can’t end on the 210th day because you can’t count to 210 by 4, 8, or 9. To make everything come out even, those weeks get extra days added to them. Each week has a name, and each day has its own name, too — but it’s a different name depending on which week you’re thinking about. So the very same day can be Luang, Menga, Pasah, Sri, Paing, Tungleh, Redite, Sri (the other one), Dangu, or Sri (the other other one). And it’s not like there’s one right answer; that day really has all those names. At the same time. And the next day has a bunch of different names too, but one fewer, because the one-day week only has one day (Luang), so every day is that day. There’s a good deal more to this, but I have to admit this is about as far as I can keep it straight. In fact I’m not at all sure I’m still quite as rational as I was when I started this paragraph. Also, I’ve just discovered it gets more difficult to type when your eyes keep crossing all by themsevels.

Tomorrow is Bruce Gaitsch’s birthday. You may not recognize the name, but he’s a musician who’s worked with the band Chicago. That’s the group that made history by issuing a double album for its very first release, in 1969. The album was a huge hit, and included several famous songs. One of them is strangely relevant to our demented little random walk today: Does Anybody Really Know What Time It Is? The idea being that if you calculate time based on what it used to be — “it’s three o’clock now because I know that exactly one hour ago it was two o’clock” — at some point you can’t find a reliable reference to the previous time, so your whole system gets to be slightly questionable. Except that there is an objective reference for counting hours. It’s not that hard to figure out when the sun is at its highest point in the sky, and that’s noon for your location. From there you can count as much as you like, and you do really know what time it is.

What about weekdays, though? They’re completely arbitrary; there’s nothing in the universe you can point to and say “it’s Thursday because of that.” And today is an excellent day to give this some thought — maybe the best day of all. Because February 6 is the day we can go back further than any other, and actually know what day it is.

In the year 79 CE, the ancient city of Pompeii was buried under megatons of ash when Mount Vesuvius erupted. Everything was frozen in time, preserved by the ash just as it was that day. Even things people had written on walls; graffiti. And sometime before the eruption — maybe years before — somebody in Pompeii had written on a wall that February 6 in the year 60 CE was a Sunday. So we do know what day of the week it is. At least what day it’s been, going back a couple millennia. You might ask why we’re taking the word of some anonymous graffiti artist for this, but look; sometimes you have to take what you can get.

And that’s what you get for today. I understand that you might feel this episode of Happened is a little bit like getting rickrolled — you know, when somebody sends you a link to what sounds like a really interesting article or website, but when you open it all you get is Rick Astley singing Never Gonna Give You Up. Really, I understand. But come on, today is not just the anniversary of the oldest known day of the week. It’s also Rick Astley’s birthday.