It is Wednesday, or as I used to say when I was in the workforce, hump day. (Get you mind out of the gutter!) It signifies the middle of the week. I did a little research and came up with the following:
here’s a reference to hump day being used in a song lyric by J.J. Cale (song is called ‘Friday’), dated 1979.
“Wednesday’s hump day, hump day’s Wednesday
Over the hump, the week’s half-gone
If I had my pay on Wednesday I’d hang out, the hump day’s gone”
Full lyrics at

HUMP DAY — “Random House Historical Dictionary of American Slang, Volume 1, H-O” by J.E. Lighter, Random House, New York, 1994, has several slang uses of the word “hump” in print going back to the 1800s. “Over the hump,” meaning beyond the midpoint or most difficult part was in print in 1914: “…Jackson & Hellyer, ‘Vocab. 46: Hump…the half-way point in a prison sentence.”
“Hump Day” — “.the day that is at the midpoint in a given period of work; (often) Wednesday, the middle of the work week. Similarly, Hump Night. 1955 AS (American Speech) XXX 226: Hump Night.Wednesday night, which is over the hump of the week. (1977 Langone ‘Life at Bottom,’ 202: Some of the parties in midwinter, that’s when you’re over the hump. Hump Night, they called it, halfway home.’) .ca 1965 in DARE (Dictionary of American Regional English): Hump day was used by counselors at summer camp to mean Wednesday.”

SO there you have it. Perhaps I will start a feature here of finding the origins of obscure phrases.