This really is simple. To be honest it is something I noticed rather than learnt.
My own 3 children are all at primary school. When they have a ‘mufti’ day it is called home clothes day.
I teach at a secondary school. When we have a mufti day it is called non school uniform day
Somewhere between year 6 and year 7 we have changed our language to make school seem a negative thing to be