I'd go with: "He likes Sunday, because he doesn't go to school that day." But I think it would be even more natural to say: "He likes Sunday, because he doesn't have to go to school."

Yan Hui Hang's right. Because is a conjunction like " but, and" . That means " BECAUSE" can be the head of a sentence, such as: "Because he doesn't go to school this day, he likes Sunday." And if not, before BECAUSE, there mustn't be dot "."

I think it should be: "He likes Sunday, because he doesn't go to school this day." Or: "Because he doesn't go to school this day, he likes Sunday."

'He likes Sunday, because he doesn't go to school on that day'. The second part is only a clause, not a separate sentence. 'this day' sounds a little odd.