Click the "First day:" field to indicate the Start date of your OOO time.

Click the "Last day:" field to indicate the End date of your OOO time.

Edit the subject line to indicate that you are OOO.

For example: Type "I'll be out of the office from _____ until _____"

Edit the body of the email, be short and concise as to who they should communicate in case they need a response during your time off.

I'll be out of the office until December 26th.

How to set up a Vacation Responder in Gmail?