I love the way the text has a border in this but it's not a standard shape. This is from an actual invitation, but my thought is that this would be on card stock about postcard type size and be the main part of the STD with the actual text.
I love the idea of keeping this basic shape and using on a variety of paper products similar to Mrs. Cupcake did in her invitation above.
On the card along with the words "Save the Date," our names, location, and the actual date, there would also be some type of mini envelope similar to the ones shown below.
As for inside the envelope... well you'll have a wait a while to find that out! But in the mean time, and guesses as to what I'm planning on hiding in the envelope?