"We don't have classes on Fridays. We play all day." or "We don't have classes on Fridays, so we play all day." Also, you write "Fridays" if you don't have classes EVERY Friday. and you write "Friday" if you don't have classes THIS Friday, however, in this context, because you mentioned the present tense, "we play all day", it implies that it is EVERY Friday, and so "Fridays" or "Friday" both work. However it is more correct to write "Fridays"

Hi! There's nothing wrong with your sentence but punctuation marks :) You'd need a full stop after "Friday." (Maybe you forgot it?) ☛ We don't have classes on Friday. We play all day. But, you can also make this one sentence by replacing the full stop I added with a semicolon. ☛ We don't have classes on Friday; we play all day. They're both accepted. Cheers!