Things that are due are owed — it's time to come up with them. Loans, library books, and school assignments usually have a due date.
The first question students usually ask when given an assignment is "When is it due?" The word due has to do with deadlines and things that are owed. When you're time is up on a library book, it's due. Many bills — such as cable and phone bills — are due once a month. If you miss a due date, you could be in trouble: you might get a bad grade on your paper or the cable guy might take away your favorite stations.