Its a day where you slap someone named Arson / Mio. For example:
“Hey Arson, do you know what day it is today?” “No?” “It’s national slap arson day” *slaps arson*
National slap arson day is on April 15th. For example:
“Hey Arson, you know what day it is right?” “No” “it’s national slap Arson day.” *slaps arson flat handed in face