I think it's because you could have found the answer in two seconds on wikipedia, and also I think it was explained somewhere in this topic.
I got that it has something to do with not buying stuff but there was no explanation as to what black friday actually is. Sure I could have looked around for it but since you were talking about it I didnt see any harm in asking you guys instead. But now I just dont care..
EDIT**
So Ryan, im an idiot for asking? WTF is wrong with you, it was an innocent question.