Where on earth did that saying come from? I don't know but in looking it up I did find this which made me laugh:
The absolute BEST day of the week, the day of maximum hope that maybe, you might make it out of this week alive. A particularly good hump day can last you the rest of the week, and by Doomday morning (Monday) you survive by anticipating hump day. Nothing goes wrong on hump day
I tend not to agree with that last bit but it's all interesting.