One of Those Days

I was at work today and heard an employee sigh heavily.  He told me, "It's just one of those days."  It got me thinking (it made my brain hurt to think so much).  Why do we say that phrase?  I understand that the phrase really means, "It's been a tough day."  However, how do we get that meaning from the word "Those"?  It doesn't make sense to me at all.  It's as if we have categorized days.  "Those days" are not good days.  We were all taught that when we were born apparently.  Maybe one day, when I'm having an awesome day, I'll tell someone I'm having "One of those days".  That will really confuse them.  Anyway, I didn't have one of those days.  I had a day, just like everyday is a day.  At this point, I do live my life one day at a time.  It is a full time 24/7 job after all.  There are no breaks in this thing we call life.