1. Go to your Gmail, and click on settings:
2. Select "See all settings"
3. In the "General" tab, scroll all the way down. You will see "Vacation responder"
4. Here, you can select to turn on the vacation reminder, and set start and end dates, what the subject of the response email will be, and what the default message is.
5. Once you have set up the reminder, click "Save Changes" at the bottom: