How can I set all holidays in Outlook to show my availability as Out of Office?

When you use the Tools, Options, Calendar Options, Add Holidays command to add holidays to your Calendar folder, Outlook sets your availability to Free, even though you'll probably be out of the office. But it doesn't take much work to change the availability for all items, and you don't have to edit each one.

Because Outlook stamps each holiday item you add with the Holiday category, you can use the built-in By Category view to identify the holiday items, then modify that view to make it easy to change your availability from Free to Out of Office. After you switch to the By Category view, right-click a column heading and choose Field Chooser. From the Field Chooser, drag the Show Time As field to the column headings. Right-click the Show Time As column heading and choose Group By This Field. Choose View, Collapse All Groups to make it easy to see the groups, then expand the Holiday group.

You should see a view like the one in Figure 2. Click in the Show Time As column for any holiday and change that field's value from Free to Out of Office. Collapse the Show Time As: Free group, and you should see a new group, Show Time As: Out of Office. To change all the Free items to Out of Office items, select the entire Show Time As: Free group and drag it to the Show Time As: Out of Office group. It might take Outlook a moment to update all the items, but you'll soon see just one group under the Categories: Holiday group, for Show Time As: Out of Office.

You'll probably want to drag the Show Time As field out of the gray Group By box at the top of the window to leave the view grouped only by the Categories field. You can then right-click the column headings in the view and toggle off the Group By box, leaving the view grouped only by Categories.