In extreme cases, you might be so tired that you end up wasting the entire weekend by doing things that are not only unproductive, but not even particularly interesting.
STEVE: Saw two movies, partied, did nothing important had fun.
JOE: Well, it was a long week, so I guess weekend syndrome is okay.
BOB: I basically spent the whole weekend playing solitaire, watching random YouTube clips, and reading random trivia on Wikipedia. So not only did I waste the whole weekend, but I didn't even waste it on anything particularly interesting.
STEVE: Dude, that's extreme weekend syndrome. What the frick did you have to do during the week?