Multiple Personality Day is an opportunity to get in touch with yourselves (not like that! get ya hand outta ya pants).
Someone with a split personality has two personalities. Someone with multiple personalities has more than two personalities. Its a psychological disorder that we hope none of our members have.
Don't be surprised to find yourself surrounded by people who are talking to themselves today. You might find yourself talking to yourself, too!
When you wish someone "Happy Multiple Personalty Day", you may need to do so multiple times, once for each personality.