The only reason non-uniform days are such a big thing is because they're unusual, so everyone digs out their best/ most show-off clothes.
I grew up without uniform. If you wore anything too outrageous, you had the piss taken out of you for the day and you learned to dress more appropriately by the next day. No one cared if we wore the same outfit every week (and as growing children that was rarely the case anyway); it didn't take clothes to distinguish between the haves and have-nots.
I can tell if a kid is poor even if they're in uniform, so can everyone else. Uniform is not a leveller. Some here argue that some may only have one or two outfits to wear - well, given the price of school uniform, they may a) be able to afford more and b) will still, perhaps, only have one or two outfits and you can easily tell if someone has to wear the same shirt three times a week.
I am a strong advocate against uniform, especially those that also dictate the students' sock colour, hair colours, hair style and body art. It gives teenagers one more thing to rebel against, parents one more thing to worry about (Will it fit? Is it washed? Will the shoes be banned next week?), teachers one more thing to deal with at work.