Well, then I suggest you send out some Save The Dates. You don't have to, but it let's your guests know when the wedding is, and that they are invited. If they need to travel for it, it let's them know in advance so they can save money and book travel plans.
When do you send them?
A rule of thumb, is about 6 months prior to the wedding. The earlier the better. I had a friend send her's out in December last year, and her wedding is this August.
What happens if the date or plans change?
It is best to make a website with all your information. That way if something changes you can go on your website and update it. A lot of people use social media sites-like Facebook that you can create your wedding page on, if you didn't want to make a website. You can also spread the word by phone, or if you wanted, you could send out another mailing that explains what's going on.
What information goes on the Save The Date?
Usually you just put the date of your wedding, and location as in the city and state unless you already have your venue picked. You can also include your engagement photo or any other photo if you are going to use your engagements for your actual wedding invitation.
Along with my blog I am starting to design Save The Dates and Announcements if anyone is interested :D
Here is a sample of one that I did...