What is the difference between Due date and Until date?

The Due date is the actual deadline.

The Until date is the date until which students can submit an assignment i.e. this is the date until which they have leeway to submit late.

1. Submissions made between the Due and Until dates are marked late.
2. Students cannot submit after the Until date.
3. If you do not specify an Until date, students can submit late until the course closes.