The first proper cold snap of the year, when tights go on under everything and you want something that doesn't need ironing, doesn't need thinking about, and still looks like you made an effort. That's when a knitted skirt comes out. Jumper weather, basically, but for your legs.
Fit is where most people get caught out, because knit moves. A skirt that feels snug but fine in the changing room can drop a size by lunchtime, especially anything with a high lycra content that hasn't got much structure to it. You want to sit down in it before you buy it, properly sit, not perch, because that's when you'll see if it rides up or bags at the back. Ribbed knits hold their shape better than a smooth flat knit, which is worth knowing if you're between sizes, since rib has a bit of built in forgiveness without going see through when you walk. Watch the waistband too. Elasticated is comfortable but can roll if it's thin, so look for one with a bit of depth to it. And check the lining situation, because an unlined knitted skirt in a lighter colour will show everything underneath it, which nobody needs to find out on a work day.
If you've got none of these, start with something mid length in a mid grey or a rust, ribbed rather than flat, with a waistband you can't see your hand through. That one colour and cut does the most jobs, work, weekend, the lot, and it won't date the way a specific textured or cable knit might after a season or two of being everywhere. The patterned or textured ones are brilliant once you already have the plain one sorted, not before.
It sits well with a roll neck tucked in, with boots that come up past the ankle so there's no gap of cold leg, and with a long cardigan over the top when the skirt alone feels too fitted for the day. Over months, knit does ask a bit more of you than other fabrics. It pills, especially anywhere it rubs, a bag strap, the inside of a coat. A fabric shaver sorts that in about two minutes and makes it look new again, so it's worth owning one rather than giving up on the skirt after the first bobble shows up. Hand wash or a cool wool cycle, dry flat, never on a hanger unless you want it a size longer by the time it's dry, which I did once and wore anyway because I'm stubborn.
It's the kind of thing that makes a plain jumper you already own look like you planned the outfit on purpose.