That sounds useful when everyone is there in person, Jane I have tried a Facebook app, but not everyone is on Facebook, so it wasn't entirely successful.
I mostly use the back-of-an-envelope system. I have printed out a calendar page for January on A4. When some one says a date is impossible I put their initials on that day with a cross, and when someone strongly prefers a specific date, their initials have a tick on that day. At some (arbitrary) point I cross out all the completely impossible days and am left with a couple of possibles. The worst-case scenario is that there are NO days possible for everyone, as has happened. Then, I put the most popular (or least unpopular) days in a hat and pick one.