Click on the Gear, top right 
Select “View All Outlook settings”
Select Mail
Select Automatic replies
Click on Turn on automatic replies
Click on "Send replies only during a time period" box
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Enter dates - Start time, End time
Click on the action box(es) that you would like to automatically do during this time period
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Type your message in the dialogue box under Send automatic replies INSIDE your orginaztion.
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If you would like OUTSIDE organizations to receive the automatic reply check the Send replies outside your organization box
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Type your message (or copy and paste from inside organization) in the dialogue box
Click Save