Once we get through Christmas and New Year, it's time for Blue Monday - traditionally the third Monday in January, when a combination of weather, debt problems, low motivation, a return to work, and abandoned New Year resolutions create a perfect storm of depression that hangs over the population. It sounds like a plausible enough theory, but don't fall for it: Blue Monday is actually a hoax.

Or to put it more accurately, it's a promotional idea dreamt up by a travel company back in 2005, as Atlas Obscura reports, when 24 January was declared the first Blue Monday. That company was Sky Travel, which enlisted the help of psychologist Cliff Arnall (then of Cardiff University) to add some credence to the idea and throw in a pseudo-scientific equation or two.

Designed by ad agency Porter Novelli, the idea was to get potential customers thinking about summer holidays to help beat the Blue Monday misery - perhaps a few weeks in the sun could help keep depression at bay? But despite it being a completely fictional nadir, Blue Monday still gets plenty of media coverage every January, so make sure you don't get caught up in the negative hype.

As The Guardian's Dean Burnett points out, depression doesn't follow a schedule, let alone one set by a marketing firm. Sky Travel has since gone out of business, so any boost in ticket sales it may have received by convincing us we were all at a low ebb obviously wasn't enough.

We do know that Seasonal Affective Disorder (SAD) is very much a real condition, as the changing seasons and shrinking number of daylight hours cause chemical changes in the brain. However, there's no such evidence that this sadness reaches its peak on Blue Monday, even if there are another 11 months to go until Christmas.

As Atlas Obscura points out, the term originally referred to the tradition of lining the church altar with blue cloth on the Monday before Lent. It's also been associated with excessive drinking and subsequent hangovers in work, and is of course also the name of one of New Order's biggest hits (though the 1983 single was about relationship problems rather than Seasonal Affective Disorder).

"True clinical depression (as opposed to a post-Christmas slump) is a far more complex condition that is affected by many factors, chronic and temporary, internal and external," writes Burnett for The Guardian. "What is extremely unlikely (i.e. impossible) is that there is a reliable set of external factors that cause depression in an entire population at the same time every year."

Even Cliff Arnall has tried to distance himself from his previous claims since 2005, but that hasn't stopped him identifying the supposed happiest day of the year as well.