Canvas has the Delay posting option for announcements. You will find this feature below the announcement text box in the Options section.

The Delay posting feature allows you to set up the announcements ahead of time and schedule them to release to participants at a later time in the semester. Please consider taking advantage of the Delay posting feature. It will allow you to get your session announcements done before it begins so you can then focus on other duties and help students get more out of your Canvas course.