Before you buy, get your hands on it properly. Hold it up to the window or the shop's brightest light and check it isn't see through when you move, because white fabric lies about its own thickness until you catch it against a bulb. Check the lining if there is one, and if there isn't, ask yourself honestly whether you're the sort of person who'll wear a slip underneath every single time or whether it'll sit in the drawer for exactly that reason. Run a thumb over the seams and the waistband for any pulling, since white shows every stress point a dark colour would hide, and have a proper sit down in the changing room, not just a glance in the mirror standing up, because that's when you find out if it creases into a crumpled mess or bounces back. For pairing, a white skirt does more work than people expect once you stop treating it like a special occasion piece. A plain t shirt and trainers makes it a Tuesday thing. A fine knit tucked in with sandals takes it into evening without changing the skirt at all. It sits well next to denim jackets, next to stripes, next to pretty much any colour you'd normally avoid wearing head to toe, because the white does the calming down for you.
It suits spring through to the end of summer best, weddings, long lunches, the odd office day when you want to feel a bit more pulled together than your actual week deserves. If you've got none and you're starting with one, go for a midi in a slightly heavier cotton rather than anything floaty and thin, something with a bit of structure that holds its shape walking and sitting both, because that's the version that survives a whole season of wear rather than one good afternoon.
Tomorrow morning I'd put mine with a striped top and the flattest sandals I own, and call it done before I've had my coffee.