Since I work every single Friday and Saturday night, I constantly hear "I bet this is the busiest time of the week." I used to assume that too. You think obviously weekend nights in the ER will be busy what with parties and crashes and such. And they are. But the busiest time of the ER is Monday night. No, really.
Here is my theory. People start feeling sick on Friday or Saturday, but not THAT sick, and they think, "I can just barely wait until Monday to see my primary care physician." So they hang on by their fingernails until Monday morning and call their own doctor, whose office staff says "we're full, but we'll call you if we have a cancellation." But they never call, because there are 45 other people also hoping to get in, so by the end of the day they realize they've already waited all weekend and aren't going to be seen anytime soon by their primary care physicians.
So they load up and come to the ER. Voilá! Monday mystery solved.