This question came up in our training this morning at CHS.
Please go to the following site which has excellent tutorials on how to accomplish this and other Zimbra features: Website - http://help.zimbra.com/.
For video specific tutorials please go to: Website - http://help.zimbra.com/videos/7x/
Here are the directions:
Please find the instructions for sending an away message for your email, and a screenshot, below:
1. Select the "Preferences" tab on the top menu
2. Select "Out of Office" from the menu on the left
3. Select the bubble next to "Send auto-reply message"
4. Fill in the message you want sent to the people receiving the email
5. Select the box next to "Time Period" and select the start and end dates
6. IMPORTANT: Select "Save"