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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
Enter dates - Start time, End time
Click on the action box(es) that you would like to automatically do during this time period
Type your message in the dialogue box under Send automatic replies INSIDE your orginaztion.
If you would like OUTSIDE organizations to receive the automatic reply check the Send replies outside your organization box
Type your message (or copy and paste from inside organization) in the dialogue box
Check your grammar and spelling
Click Save