Once logged into your webmail, look for the options menu in the top-right corner of the screen and select Preferences.

In the preferences page, first select the Filtering tab, then select the Configure button to the immediate right of the Holiday autoresponder row.

Start by checking off Enable responder when you're ready for the system to begin responding on your behalf. Then enter your message subject and body. You also have additional options here like choosing the enable the responder on the certain date, or having it auto-disable a set number of days after activating. When you're happy with your selections, press the Save button at the bottom of the screen.

Don't forget to come back to this page after returning and unchecking the Enable responder box if you haven't specified the Disable this response after ___ days field.