At my prior employer, I was able to mark agents on Vacation when they were out on holiday, but now with my new employer I cannot find this feature. We have one agent who is out of the country for a few weeks and tickets keep getting assigned to him. Where did the vacation check box go?!
If you have the Helpdesk agent role assigned and that role has the Access Calendar and Alter Calendar Events privileges enabled, you can view and change calendar events.
This might be the feature you are looking for.