On the web app of SeriousMD
You just go to Appointments > Month or Week view.
Then hit the + button
Select Event
On the popup, enter your event's name and time/date (Select All Day if you'll be away for days)
Next time you try to schedule a patient, you will see a warning to alert you so won't accidentally schedule a patient on that date.
On the iOS app
Just go to Calendar
Tap the + button
Add your event