There could be a couple of reasons it might be considered the worst day.
It is known as hump day and if you didn't get humped, you might feel sad.
It is on the teeter point of not being done with the whole week or being elated your over the hump!
However you would like to think it, is up to you!
I say it's a definite
"SHOT DAY!!"
You don't have to think about it in any way that way!!
;) :)