Every assignment has a start date and a due date. Students cannot see their assignments until the start date has arrived. You can review an assignment's start date on the Assignments page. If it is the the future, students cannot see it. If it is today or in the past, students can see it.
You can edit the start date at any time by clicking the pencil icon in the top-right corner of the assignment (or on the assignment's dashboard).
Students can see and submit an assignment after the due date has passed.