We did the legal ceremony one year before our real wedding (Jan 12). So our marriage certificate says Jan 12, 2011, and our wedding is Jan 12, 2012.
We invited a witness each (and their spouses) and had a little ceremony in our livingroom, took less than 5 minutes, it was nice - but not my wedding - although I did get a little cake, because I love cake.
I think most people know we have to do the legal stuff beforehand, anyone who hasn't asked, probably just assumed we'll do what we're supposed to make it legal. Luckily I haven't been asked that question to much!