In addition to setting a due date and time for an assignment, instructors can specify a specific date range when students can view and submit the assignment. These dates are called availability dates. These dates are optional and can be set depending on how you want to manage the assignment.
The due date is the date and time when the assignment is due. Due dates are not required in Canvas (since the alta due date and time will be visible to students), but they are helpful in managing course workflow and deadlines.
If you want to create a date range for students to view and submit an assignment, you can set availability dates. Availability dates can also be called lock dates. Once the available date has passed, a student will not be able to access the assignment. This also locks them from being able to keep going in an unfinished assignment, practice more in an already completed assignment, or review previous learning activity (such as completed instruction and questions). You should only enable availability dates if you wish to completely lock students out of an assignment before or after specific dates and times.
If you're concerned with allowing students to receive credit after a due date, you may wish to change the grading settings in your course to not allow late completion. Read more about late completion and penalty policies here.