For more on the difference between due dates and availability dates, click here.
Starting in the course platform, click on "Modules" on the left-hand side course menu.
Click on the name of the assignment or assessment you'd like to edit to open the item's page.
From the item's page, click "Edit" on the top right corner.
Scroll down until you reach the "Assign" section.
Set or change the due date or availability date of the item.
To add additional dates, click the "+Add" button at the bottom of the box.
Click the "Save" button when done.
Please remember that your students' account time zones need to match up to the course time zone in order for this function to work with precision. For further information, please see this article on time zones.