FWIW, I think all day sessions are great. But...
...that said, I personally believe they run in conflict with some of the most attractive aspects of a Gameday. These events are perfect forums for meeting and gaming with new people, experiencing different styles of GMing a session, and experimenting with different game systems/worlds.
Perhaps the adventure can be partially abbreviated, or run over two separate Gameday events. I know the latter option is somewhat complicated, but I have seen this done with encouraging results. Posting the event two cover two entire time slots is fine (and will probably be a great game), but it may cost you several players interested in exploring other options.
Either way good luck, I'm sure this will work out just fine!