Definitely earlier than the 1980s. Google Books is an awkward place to search, since it often dates periodicals by the first issue in their records, but a few hits seem to be solid.
The hard hat girl, power engineer: a novel - Page 84
McCulloch Byers - 1976 - Snippet view
Well, with less than a half hour left now, it looked as though she might pull through her third day in fairly good shape. The third time’s a charm, they say; and today must be the normal, “routine”, office day, — Wednesday, — “hump day” — over …
California: The New Society - Page 2
Remi A. Nadeau - 1974 - Snippet view - More editions
In the average California office, Wednesday is “hump day”— you’re over the hump. By Friday, you’re mentally already on the weekend. Friday night the going-home traffic is clogged not only with commuters, but with house trailers, campers, …
Texas Monthly - Jun 1974 - Page 117
Vol. 2, No. 6 - Magazine - Full view
Weekdays: 11:30-2, 5:30-11. :30. Weekends: 5:30-midnight. Moderate. Jaxon’s, 508 N. Stanton (532-0950). Homemade soups, salads and sandwiches. An afterwork “in” spot, the court is packed for Over the Hump Day (Wed) and T.G.I.F. Bar.
Current Slang - Volumes 2-5
1967 - Snippet view - More editions
The middle of the week, “hump day.” 1, 3. , v. To act (command to get busy) . Humus , adj. Admirable or beautiful, “tough” or “cool.” Hurt , adj. Of limited sophistication (applied by those of limited sophistication. —I, 1. . . The remainder of the …
Proceedings, Annual Great Lakes Park Institute - Issue 21 - Page 85
1967 - Snippet view - More editions
This is Thursday; over- the -hump day in your Institute. You have had three busy days and evenings. You are now reaching the time when you do not wish to listen to a long dissertation on the history of the preservation movement - - it goes …
An introduction to school administration: selected readings - Page 30
Mervin Chester Nolte - 1966 - Snippet view - More editions
THE SON OF A MEMBER OF THE SCHOOL BOARD Terrel Howard Bell One morning, after the third week of school had passed the mark that the teachers called “hump day,” or Wednesday, a very small boy came to school. He registered in …
California: the new society - Page 2
Remi A. Nadeau - 1963 - Snippet view - More editions
In the average California office, Wednesday is “hump day”— you’re over the hump. By Friday, you’re mentally already on the weekend. Friday night the going-home traffic is clogged not only with commuters, but with house trailers, campers, …
As you can see, California: the new society went through multiple editions, so somebody would need to check the actual book to see whether it had the usage from the beginning.