Just because you don't have the details of your event ready doesn't mean you can't let your guests know to save the date. And with modern technology allowing anyone to create high quality documents, there's no reason that your save the date postcards can't be as special as the invitations you end up sending out.
In fact, with the same modern technology bringing prices down, you can also get your save the date postcards done by professional printing services provided by PostcardPrinting.com that allow you to create your own postcards using templates, fonts and images we provide.
Be sure to include as much information as you have about your special event, so that your guests know whether to save their day or night and what kind of an event they should expect.