It Has To Be はず

だろう marks an open guess, かもしれない marks a bare possibility, but はず sits above both, a conclusion you’re confident in because you have a specific reason to believe it, a schedule, a pattern, something you know.

な-adjectives and nouns insert な before はず, 暇なはず, just as they do before の or のに, while い-adjectives and verbs attach directly. The reasoning behind はず is often spelled out explicitly with から right before it, as both examples above show.