How do I black out holidays for my webinar?

Holiday at automated event schedule is a useful tool, which can also be used to advance a webinar start date until such time as you're ready to start marketing your webinar event, or for Holidays as intended.

In any case Holiday is a fairly simple item to setup under Scheduling seciton > Advanced Scheduling.

To select a single day, select it for both Start Date and End Date, or pick through mulitple days at schedule (as shown below) This will remove those days selected from being available at Registrations Pick a Time scheduler.