Yup. It's Hump Day — otherwise known as "Wednesday" and while that name is still printed on our calendars, the former has taken over in everyday conversation. And no — this wasn't something millennials thought up while YOLO'ing with their baes.
"Hump Day" is a play off the idiom "over the hump," which refers to being at the midpoint. The phrase was used colloquially in the 1920s — when people were saying things like "applesauce" and the "bee's knees"...
By the 1950s people in North America began referring to Wednesday informally as "hump day."
It started being used more in more in subsequent decades but still required clarification as it hadn't caught on.
Most of us probably didn't get the unofficial memo until Geico brought us their "Hump Day Camel" ads.