Vacation mode prevents people from purchasing products, while still being able to browse your store!
You can enable Vaction Mode in Website > Settings under the "Sitewide Messaging and Vacation Mode" section.
Directly below vacation mode, you can enbable a sitewide message. This goes very well with vacation mode, as you can make it clear that your site is on vacation mode, as well as let your customers know how long you will be on vacation.
Vacation mode is not required to have a sitewide message, you may enable the sitewide message at any time and have it contain whatever you like!