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1.
A day that youself,or your friends take off from work/school(which day off from school is more likely)and devote that day to jacking off.
Bob: "Hey did you see where chris was today?"
Peter: "You didnt hear, He is having a jack off day."
Bob: "Boy I need one of those soon."
by Jack mehoff12312 September 20, 2007
 
2.
Jack Off day is on the 5th August each day.

Originally coined when an employee added to his calendar that 4th of August was 'Jack Off' day.

Jack off day is a special day of each year that brings joy and happiness to all that follow it. The real magic in Jack Off day is those who do not know about it wouldn't suspect a thing going on.

Followers of twitter tend to tweet with the tag:
#jackoffday2010
#jackoffday2011
etc
So you know who they are...
Ben: Andy seems strangely normal today
Lloyd: Yeah I believe he's celebrating something called Jack Off day
Ben: ...yeah, whatever that is
Andy: *inner joy*
by brightpearl August 05, 2010