In which keeping track of the days of the week is surprisingly difficult

By which I mean: I've been learning Turkish for... three years now? And have only, in the past fortnight, finally got my head around the days of the week. Which go like this: pazar (Sunday), pazartesi (Monday), salı (Tuesday), çarşamba (Wednesday), perşembe (Thursday), cuma (Friday) and cumartesi (Saturday).

Perhaps unsurprisingly, I had particular difficulty with salı, çarşamba and perşembe. And then, a fortnight ago, I noticed that çarşamba and perşembe are the same word with a different prefix, and everything suddenly got! a lot easier!

Because, see, one of the things Turkish does, in addition to being agglutinative, is so-genannte vowel harmony, whereby the vowels in various bits that routinely get stuck on the ends of other words change (between either two options or four options, depending) based on the last vowel in the root word (which in turn depends on its preceding vowel, in many cases). So both çarşamba and perşembe are built up using the component -şImbI, where capital dotless I indicates "a vowel harmony goes here".

I... genuinely feel like it took me an embarrassingly long time to spot that, given the givens.

Anyway, I badgered the friend who's the reason I'm learning Turkish, who kindly explained to me that what's going on here is that şImbI is from the Persian shanbe, being the word that is also "Sabbath", and which means (for these purposes) "first day of the week". The prefices "char" and "penj" (via A Phonology) are "four" and "five", as in, that many days post-Sabbath.

So now I actually know the days of the week in Turkish, too, and they're reasonably solid, and I'm very pleased about this.