Click on any piece of content in your course. It could be an assignment, quiz, discussion, PDF, or other file. Scroll down to the section called "Activity Details." Click the round switch to make content Visible and Invisible to students. You can also add a start date (when the content becomes available), a due date, and an end date (when the content disappears for students). When you are done, scroll down and click on the blue "Update" button.