You can also set up an auto-reply based on the time you are unavailable. If you're not sure when to schedule your out-of-office message, you can also set up an auto-responded. You can even have it set up for specific days in advance, so that you don't have to worry about it. It's worth the time to do this. You can even customize it, but make sure to include a custom out-of-office message in it.