The 15th day of the month of May where you can smash all your broken stuff that has become worthless throughout the year. Or to smash things for fun because you like to break stuff. May 15th was chosen because a YouTube user posted a video saying May 15th was 'Smash Your iPod Day'. Since iPods aren't the only electronics to smash, or the only type of stuff to smash; That is why May 15th is declared Smashing Day. Not to mention the proximity to other holidays on the calendar is far enough away that May 15th is a great date.
Guy #1: Why do you have all these broken electronics stored in your garage.
Guy #2: I am saving them to smash up on Smashing Day, which is on May 15th. Want to smash things with me?
Guy #1: That sound great, I would love to celebrate Smashing Day with you! Lets plan for it.