That depends on the context: Weißt du, ob wir Montag frei haben? - Do you know whether we have Monday off? (The speaker does not know whether Monday is off or not.) Weißt du, dass wir Montag frei haben? - Do you know that we have Monday off? (The speaker knows that Monday is off, and is asking if the addressed person knows that) Here the meaning is very similar, but in other situations there may be more of a difference.