I am generally in favor of open carry, that being said here are some problems I have with this idea.
1. Most Halloween parties that I go to serve alcohol, Guns+Alcohol=
. Even if you aren't drinking, some one else could get drunk and think it would be funny to grab your gun.
2. Police deal with LOTS of wierd sh$T on Halloween and giving them one more thing to be freaked out about is just asking for trouble.
3. The confusion between REAL and FAKE. This is a time of frivolity, where people assume that what you are wearing is a costume and not real. Unless you are at a private party with other "Gun People" and every one knows that the gun is REAL and knows how to respect that, most people will see your gun and assume it is fake or unloaded.
Open carry on normal days it fine, people will see it, recognize it as real, and leave it alone. Open carry on carnival days is a bad idea because some people will see it, NOT recognize it as real and could act in a dangerous manner because of the that false assumption.
Unless you can ensure 100% weapon retention and plan to stay away from drunk/crazy people and police officers, then I would stick with a concealed weapon (out of sight, out of mind).