Admin Steps:1. In Setup, go to Chatter Settings.

2. Click Edit.

3. In the Out of Office section, enable Users can set Out of Office messages.

4. Click Save.

User Steps:

1. Go to People tab and select your name.

2. Click Out of Office button.

3. Check Set Out-of-Office message.

4. Enter Start Date, End Date and Message.

5. Click Save button.

Output: