The time when an assignment, project, or application "is due" is the time that it must be finished by. The idea is very similar to a deadline. However, "deadline" is a noun and "due" is an adjective.

You can tell what the time something is "due" is by saying:

• due on (a day)
  

  I think it's due on the 23rd.

• due by (a time)
  

  It was due by midnight last night.

• due (next ___/last ___)
  

  Your application is due next Tuesday.