On the iPad/iPhone, just go to Calendar, then hit the Plus button

This popup will appear.
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Enter the name of the event, like “No Clinic” then set the date. If you’ll be out for the whole day, just turn on “All Day” like in the image above.

Hit the save button and it's now saved.

So next time you try to schedule a patient for that day, a warning will show up to indicate the schedule conflict.

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On the web app:

It's similar to the iOS instructions above.