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SP day is every 28th day in each month and it's a day, when fans from all over the world celebrate Simple Plan.
The idea started in the SP Crew, where this date (the 28th) was set randomly, just in order to have a date to celebrate. And what's the purpose of the SP day? Wearing SP clothing, listening to SP's music, talking about SP and just doing everything in your power to let the world know, that you are a Simple Plan fan!
-"Hey! Happy SP Day all!!!"

-"Guess what?! It's 28th June!! And it's an SP Day!!!"
by Syeren MCRmy_SP May 24, 2009
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