For example, today is Wednesday, February 14, 2007. To me, the idea of "Next Friday" would be the first friday after today, which would Friday, February 16, 2007.
But no. That day is "This Friday", and "Next Friday" is the 23rd.
So, what do I call the 21st? Is it "Next Wednesday"?
Similarly, I would call Monday, February 12, 2007 "This Monday", because it is in the past, and during the current week. But "This Monday" is really the 19th, and the 12th is "Last Monday".
This all has been explained to me many times, but from time to time, it still trips me up.