Where does the seven-day week come from and why does it govern our lives?

From the dread of the Sunday blues to the boredom of a Wednesday hump day to the sweet relief of a Friday afternoon, the seven days of the week govern our lives in countless invisible ways.

But while other units of time, like days, months and years match the Earth's rhythms, weeks are much more of a human construction.

"It's the one unit of time that actually isn't visually perceptible in the natural world. And yet … it has a 'real' presence in people's lives," David M Henkin, a history professor at the University of California, Berkeley, tells ABC RN's Late Night Live.

In his new book, The Week: A History of the Unnatural Rhythms That Made Us Who We Are, Professor Henkin unpacks exactly why we don't like Mondays.

Seven planets, seven days

The origins of our Monday to Sunday week stretch back millennia.

"Some people have argued that there are roots of this in Babylonia [which lasted from around 1900 BCE – 500 BCE] but the evidence for that is pretty slim … The Romans definitely had [a seven-day week]," Professor Henkin says.

The ancient Romans settled on a seven-day week around 2,000 years ago because there are seven celestial bodies that can be seen from Earth: the sun, the moon and the five planets.

"The idea was that each of those celestial bodies had dominion over a day," Professor Henkin says.

Also around this time, Jews were marking the Sabbath every seven days, with Christians later counting seven-day cycles (and Muslims also doing so beyond that).

Professor Henkin says the Roman and religious structures "became entangled or unified, they converged to form a calendar unit."

"Since then, it became a meaningful timekeeping device, a dating system, in most of the societies that were touched by the Roman Empire," he says.

The Roman Empire faded, but the major monotheistic religions spread, taking the seven-day week with them.

The 'birth' of the modern week

In the centuries that followed, the only major features moored to a particular day within a seven-day week were a day of worship or rest.

Apart from these, the time unit and various days within ultimately weren't all that important to most people.

Medieval knights, for example, probably didn't care if it was a Monday or a Thursday ― it was all the same to them.

But Professor Henkin says this dramatically changed in the 19th century, when the "modern week" came into being.

Around this time, specific days of the week started to be used to schedule and coordinate both social and business activities.

All manner of events — from payday to receiving mail to doing laundry — started to become attached to a particular day of the week.

Also in the 1800s, the concept of a two-day weekend started to take shape for some portions of society.

"I spent many, many years reading people's diaries, looking at people's lives and trying to figure out when, whether and why it mattered to them what day of the week it was," Professor Henkin says.

"[It was in the 19th century when] suddenly, people began to recall events from the recent past, using the day and not the date to tell you when it happened."

Professor Henkin says the rise of weekly fixtures and weekly plans meant people started to talk about "feeling" this unit of measurement for the first time.

The role of TV

Professor Henkin points to one technological development that further established the seven-day week as we know it: TV.

In the mid-20th century, televisions became ubiquitous in people's homes across the world.

"Television was important, especially in the period after World War II, in coordinating people's weekly habits," he says.

"[Initially] there were very few options of what to consume, and most people in the US were consuming typically only one of three offerings on a given night. The same was true in Australia. It really meant that people's weekly habits were far more coordinated."

"In the 1960s and 1970s … people's Wednesdays and Thursdays were probably more shaped by what they watched on television than by any other single consideration."

So the societal shifts of the 19th century and the ubiquitous technological developments of the 20th century cemented the rhythms and the importance of the seven-day week.

Alternatives to the seven-day week

Over the centuries, there has been opposition to the seven-day week, with some countries changing it.

After the French Revolution, the victorious revolutionaries implemented a new calendar with a 10-day week, to both shed old religious influences and to decimalise the measurement.

In the 1920s and 1930s, the Soviets introduced a five-day week and then later a six-day week, with different workers having different days off.

Needless to say, the French and Soviet models failed, with both returning to the standard seven-day week.

From the early-to-mid 20th century, some in the business world also pushed to change how we use the week, as it does not fit evenly into months or years, and therefore creates headaches for bookkeeping.

"[This movement] was not attempting to change the length of the week, but change how the week fits into other calendar units," Professor Henkin says.

British calendar reformer Moses B Cotsworth proposed the International Fixed Calendar, which was made up of 13 months with 28 days each. And in his calendar, each date always fell on the same day of the week, so, for example, November 25 was always a Sunday.

"It seemed like a ripe moment for the international community to 'tame' the week and make it fit into all the other time units."

But the International Fixed Calendar was not adopted, after it failed to win final approval from the League of Nations in 1937.

Although there was one notable exception, the Kodak Company, which used it for bookkeeping from 1928 until 1989, when the company aligned with the rest of the business world.

Is the week fading?

So what next for the seven-day week?

"I think it's not quite as unifying a calendar unit as it was 30 years ago. And as a mnemonic device, it's not quite as necessary or as useful," Professor Henkin says.

With far more flexible work arrangements, on-demand entertainment and much of our lives being lived online, we're far less beholden to what day of the week it is and what that prescribes.

But Professor Henkin says the recent COVID-19 lockdowns showed we still form so much of our routine based on what day it is, even when confined to the home.

"I actually don't think that [the week] is unravelling. I don't think that we're going to get rid of it," he says.

And so, for the foreseeable future, many of us probably still won't like Mondays.