This is anecdotal and I don't have data to support my claim (although someone can probably find something on Google).
You've never heard "Thursday's the new Friday"? College kids and young employeds party hearty on Thursdays now. It started in the early 2000's.
Uncle Google shows that even the NYT knows about it: http://www.google.com/search?q=thursdays%20the%20new%20frida...
Remember: just because you can't figure out the reason intuitively, and immediately, doesn't mean there isn't one.
It just occurred to me that there are probably reading this who have no idea what it was like when your schedule was entirely set by your employer with no wiggle room. Yikes!
Silly choice of graph type.
I guess people there like to sleep in.