The adverb o’clock is colloquially and humorously used with modifying words—as in expressions such as stupid o’clock, which denote an unreasonably, excessively or inconveniently early or late hour.

This adverb is also used with nouns such as beer to suggest a time at which it is considered acceptable or sociable to start drinking the type of alcohol specified.

The expression beer o’clock has been used of various times of the day (cf. additional notes on ‘beer o’clock’) but in particular of 5 p.m. as the end of the working day.

It was apparently Stephen King (born 1947), U.S. author of novels of horror and suspense, who first used the expression beer o’clock in the sense of 5 p.m. as the end of the working day. The earliest occurrence that I have found is from King of Horror: The Master of Pop Dread writes on . . . and on . . . and on . . . and on, an article about Stephen King, by Stefan Kanfer, published in the U.S. magazine Time on 6^th October 1986:

Every day at 9 a.m., except for his birthday, the Fourth of July and Christmas, a 6-ft. 4-in., 198-lb. creature climbs into a T shirt and jeans, swallows a vitamin pill, drinks a glass of Maine tap water and turns on some hard rock on WZON. He is never dissatisfied with what he hears: after all, he owns the station. With a few breaks, he will type until what he calls “beer o’clock” — about 5 p.m.

On 21^st September 1991 (incidentally Stephen King’s birthday), several U.S. newspapers published the following; for example, the Kokomo Tribune (Kokomo, Indiana):

Today’s quote: “(I work until) beer o’clock,” Stephen King, on his 9-to-5 writing day.

Another early instance of beer o’clock, again in the sense of 5 p.m. as the end of the working day, is from Beer tax may be hard to swallow, by Kevin Maney, published in the Argus Leader (Sioux Falls, South Dakota) on 18^th June 1993; the article is about “the hottest issue among the 750,000 members of the Beer Drinkers of America”:

Now the Beer Drinkers have worked themselves into a froth over proposed increases in the federal beer tax.
Hillary Rodham Clinton’s task force* has said that one way to help pay for health care reforms would be to radically hike “sin taxes” — federal excise taxes on cigarettes and alcohol. The argument goes that people who smoke and drink are bigger burdens on the country’s health care system, so why not make them poor, too.
One proposal would raise beer taxes 150 percent, which would be in addition to lots of recent beer tax increases on federal and state levels. Federal beer taxes would rise from $1.31 a case to $3.24 a case, the Beer Drinkers say — about 8 cents a beer.
To me, that wouldn’t hurt much. To my colleague Jim — who points to his watch every day at 5 and says, “It’s beer o’clock!” — Clinton might as well attach a funnel to his checking account.

* The Task Force on National Health Care Reform, headed by Hillary Clinton (born 1947), then First Lady, had been set up by Bill Clinton (born 1946), 42^nd President of the United States.