Enable Out of Office Message in Chatter:

• From Setup enter “Chatter Settings” in the Quick Find box
• Select “Chatter Settings”
• Check “Users can set Out of Office messages”
• Save

Set an Out of Office Message in Chatter:

• Open your profile
• In your profile page, click on “Out of Office”.

• Enable Set Out of Office message.
• Select the start date for the time you’ll be out of the office. The start date is just to let people know when you’ll be out. The message starts showing up next to your name as soon as you save it. It doesn’t wait for the start date.
• Select the end date. Your message expires after the end date.
• Leave the message as it is, or customize it.
• Click Save.