A few things in defence of schools. We have all our dates to do with learning, assessment, training, meetings set in advance- like someone compaired to their board meetings- you know, the actual job of the school that most parents don't see any of. Then, on top of the 60-70 hr week that the average teacher is doing, we try to fit in things like this to 1) raise money for charity 2) make school more fun than endless tests the government demand 3) teach children the importance of charity.
It hadn't been planned months in advance as it usually takes a volunteer/pta member to sort it and they've just finished organising the Halloween disco/harvest collection/ remembrance day assembly/ started the nativity shite.
Speak to a lot of teachers and they'll usually say, "Oh no! Dressing up day on Friday- may as well cancel the lessons I've planned as the kids will all be crazy"
As for expectations from school. I expect the following from my class: 5 in normal uniform, 10 in own clothes (no theme), 5 in spotty-ish t-shirt (Primark,George, etc) and jeans, 3 made outfits (probably jeans and a white t-shirt with spots drawn on). No child without a spotty outfit will be commented on in any way. If we'd asked for cakes too, I'd be saying a huge thank you to 5 children (and their parents) who brought something in - one would be home baked, the other 4 value range cakes.
Don't get me wrong, I can totally see why parents hate it, especially the short notice- we'd love nothing more than time to plan these things well in advance (and I'm sure there's some fab -well funded- schools with the admin staff to do this). However, most of the time we're completely bogged down with the day-to-day teaching/planning/marking. But I would only ever voice how much I bloody hate non-uniform days anonymous online cuz otherwise I sound like an uncharitable, moaning teacher in real life too 