My girlfriend and I did hastily organize a Mystery dinner one year ago. It was pretty open-format, where every player got some objectives and intelligence and had to achieve them. We downloaded most of it from an Italian website - I had found some English ones that had better infrastructure (ready player letters, items and so on) but I didn't have the time to do the translating job, so we went with it. It went over pretty well, the plot didn't have a "starting crime", but we pretty soon had some drug contraband, art thievery and murder going on. (not to mention family drama)
Some suggestions:
- Don't try to organize both the game and the dinner, delegate away the dinner if you can.
(you can have the guests bring their own food, for example)
- Explain well the objectives, rules and stuff to the players, possibly in written form since you won't have time to do a debriefing to each player before dinner starts. I told the guests what kind of character they had so that they could dress accordingly, but I didn't tell them the objectives and secrets to avoid spoilers (thay all knew each other).
- Be ready to make up rules as you go - this is pretty much like DungeonMastering: the players will come to you with all sorts of weird ideas, and you'll have to decide what is ok and what is not, and how to handle it.
- Fit the characters to the players
- You won't be able to organize it in less than a week, and I suggest you take more time, really.
Good luck!
PS: The only site I can remember looking at is
http://www.freeformgames.com/. They have a couple games that are free to download, and they provide you with almost everything you need.
