This feature needs to be enabled from Setup menu, looks for Chatter Settings, scroll down to Out of Office section, then enable Users can set Out of Office messages.
Once this enabled, all users can go to their Chatter profile, by clicking their icon in the top right area, then click their name, to enable and set their Out of Office Start Date, End Date, and custom Message.
click icon -- name
set Out of Office
Once the message set, within the start and end date, message display prominently next to the user name in Chatter.
Reference: Take a Guilt-Free Vacation with Out of Office