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1.
A day that comes once every month to two months where Joey, an administrator at Hocus*Pocus, pulls a mass-scale prank, which usually involves spam-mailing people.

As of writing this definiton, only 2 Prank Days have occured:

Prank Day 2003- Mass emailing of McDonalds
Prank Day 2004- Mass emailing of Microsoft under name of President
"2 days until Prank Day"
by ShadowMan August 07, 2004