As seen above you can access this in the settings tab when first creating the webinar

The below image shows how to add it in afterwards

1: go to webinars
2: select section for last saved webinar in question
3: click on webinar or click 3 dots / more to edit

From here you can click on Edit schedule button

1: click on settings
2: make sure it is toggled on and add in your dates to black out
3: update schedule to save those dates