What is wash day? what are the advantages and dis advantages of having a wash day in school?

Wash day or mufti day in my country is a certain day where one is free to wear anything casual. Or simply not a regular uniform day in other words. Usually it is being done to help ease the burden of those students with only 2 sets of uniforms. In my country, wash day is usually wednesday so that the students can was their uniforms and be prepared for it the following day