In Selling Manager (if you're subscribed), at the bottom-right of the page is a list of links for different functions. (Under Manage my Store).
You want Holiday Settings - click on that then in the list, tick the Holiday Settings to On, then set the date and customise an auto-reply message for anyone that contacts you while you're away. You can also select to have your listings hidden so that they won't appear in search results, and anyone attempting to buy (from their watch list) will have the transaction blocked - recommended if you'll be away longer than a few days as the notice is often missed, which can lead to poor feedback over postage times etc.
