Christmas jumper day is a pain in the backside. Along with 'odd sock day', 'wear yellow for anti-bullying', Children in Need wear a spotty top day, blah blah bollocks.
Yes you can get them from charity shops or vinted/whatever. Not the point. It's not about the cost for me (fortunately we're ok to buy them and donate old ones) but it's just one more thing to do and remember at a crazy busy time of year.
Dress up/different clothes day are often a nightmare for children with SEND. With most schools running at 30-40% SEND cohort these days I'm surprised they don't cross-check their absences in this cohort against dress up days. I've done the math here (for work) and there is always a higher rate of absence than usual across children with SEND on days where dress up is an 'option'. The change to the norm is incredible stressful and overwhelming for lots of them and for some it means they're unable to attend at all.
That's before you even get to those families who can't afford it, or are too time poor to organise whatever top/socks/costume that's asked for this time. Which is almost never the same year on year anyway (with Christmas jumper day being an exception).
And yes of course it's never compulsory but then the children who 'choose' (it's not usually a choice it's usually out of necessity or that they actually can't) not to participate are often sad and feel the odd one out.
We most of us agree to school uniform when we send our children to school and comply with set uniform rules all year. So let's just all agree to stick to those please! There's no need for costumes etc.
If they want to be festive they can spend some time in class during the last week when they do fun activities nearly all week making some paper hats or something.
I'm not a fan, can you tell?