Log into your Outlook account via the Web: https://login.microsoftonline.com

On the top navigation bar on the left-hand side click on Outlook.

On the top navigation bar in the far right hand corner click on the gear icon.

Then select View all Outlook settings on the bottom.

Select Automatic replies.

Turn on Automatic replies, enter dates/times for this period and your stated automatic reply inside your organization and if you'd like outside as well: