[KyleGoetz]

Assuming you mean, say, next week you have one of those days off:
来週、水曜日か木曜日かは休日だ。 (I think this one sounds slightly more casual with か as "or")
来週、水曜日あるいは木曜日は休日だ。 (I think this construction with あるいは as "either ~ or ~" sounds more high-level/professional/whatever)

Of course, other things like だ／です need to be taken into account, too, based on context.