The realization that summer is coming closer to an end. During the last week of Summer you try and cram everything from the list in, but most likely miss. Often Summertime Blues occur during the days preceding labor day, and as such mark a saddening time on a persons' life - this is because after labor day basically summer is over. Everyone after labor day returns to their prior engagements such as work or school. As we know, summer is the best season because you can do nearly everything and nothing. There is no official cure for the summer time blues, as it's inevitable.
The only way to temporarily mark the occasion is to party hard on the night of labor day, and try to cram some things from the summer list in. Also re-enacting something you did to kick-off summer helps or you can just reassure yourself that next summer is coming soon.
Also generally the only people who like going back to school during this time have had very bad summers - they did nothing, so they're summer haters. It's like being a gay kid in a hick town that you have to go to in the summer or a hardcore gamer whose had nothing or no one to go outside and play with, they can't wait for it to be over.
R.I.P. Summer of 08
Jim: What's wrong?
Frank: It's. It's Summer you know?
Frank: Well it's basically over and-
Jim: And you got the Summertime blues.