Some people think that it's fun to double up a birthday party as a costume party. This can go over well in certain circumstances, but not everyone relishes donning a toga all night to help you celebrate your birthday Caligula-style. While you may think it's fun, there are many adult party poopers who won't even dress up for Halloween anymore. Putting pressure on these fuddy-duddies to throw together a '70s disco costume might keep a few of them from showing up. So, unless all of your friends belong to a theater troupe, save the costume parties for Oct. 31.