It means "National Punch Dan P. in the Face Day." It is a national holiday at a place on the Northeast Seaboard. Big wonder why it is not an international (or universal) holiday yet. Don't live without celebrating it. (I) can't give you enough reasons to celebrate. Really, all of us need to rejoice.
Celebrate NPDPITFD, or be square.
Shark tanks are optional, but recommended.
Baseball bats are mandatory.