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Using the factors as defined in the equation, Arnall determined that Blue Monday should mathematically fall on the Monday of the last full week of January. In 2013, this is 21 January.

Most real scientists scoff at the idea of Blue Monday, and question Dr Arnall's credentials. This time last year, Cardiff University psychologist Dean Burnett told the Guardian: "Dr Arnall briefly taught some psychology-related evening classes at the university's adult education centre. Apparently, this makes him a Cardiff University psychologist. Using that logic, I'm an Asda manager because I once made one of their staff fetch me a discount chicken."

Bright Dress Code for Blue Monday

In reaction to the all-pervading gloom, Mental Health Research UK (MHRUK) is running a well-being and awareness campaign - 'Blooming Monday' - encouraging people to wear their brightest clothes to raise awareness of depression and seasonal affective disorder (SAD).

"We hope that businesses, colleges and organisations will take part in Blooming Monday on 21 January. Down with drab, dreary colours. This Monday let's brighten up Britain," Dr Laura Davidson, mental health barrister and trustee of MHRUK, said.

"This is not just about awareness of seasonal affective disorder and depression, but it's also a well-being campaign. Whilst wearing bright colours will not prevent SAD, which is caused by a lack of sunlight, there has been plenty of research linking mood and colour," she added.

Other sites devoted to the phenomenon include Bluemonday.org, which encourages people to hold up signs explaining why things aren't so bad after all, and sending a man dressed in blue on a tour of educational establishments.