The four primary Vacation periods (Thanksgiving, Christmas, Midwinter, and Spring) are available for editing on this Tab. The default dates that you see on the initial launch are only suggestions; you use the date pickers to alter them to match your school's actual vacation time periods.
Understand that the dates you set up here are inclusive; when you click Done (located on the Main Tab), the Start and End dates of each vacation period are included as a day of that particular vacation.