The Holiday feature in the Internal/Admin calendar is used to tell the system when classes will not be held for an observed holiday or day of closure for your organization. If a student is enrolled after the holiday has occurred and the class is still in session, the class will be prorated to reflect the fact that the class was not held on that date (if you have prorate turned on).
In the left nav panel, select 1 Configure | Calendar. From the drop down, select 2 Holiday. Type a 3 Name for the Holiday and select the 4 specific date for the Holiday. If you are scheduling more than one Holiday, for example Thanksgiving Day and the day after Thanksgiving, that would be two separate holidays. Type any 5 Details for the Holiday (optional). Save the Holiday. Now any classes you schedule that fall on a Holiday will exclude the Holiday from the scheduled meeting dates.
Don't confuse the internal/admin calendar with the calendar your customers can view on your public site. They display different things and serve different purposes. You can learn more about Calendar Notices and News Items later.