To book a vacation go to the Settings tab on the left hand side menu.
Under the Your Settings title select Hours/Vacation.
This will now show your settings options on the right hand side.
Under the Vacations header click the "add new vacation" button.
This will pop up a new window where you can fill in the start and end date of your vacation. HIt save and you're all set. Your vacation will now be booked off for you in the calendar.