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.