So it's Blue Monday again - or is it? And what does that mean?

It is generally thought of as falling on the third Monday of January (i.e. today), but some people claim it is the second or fourth, or even the Monday of the last full week of the month.

The concept was created in 2005 for a Sky Travel press release - the idea being that the glummest day of the year would be a great time to plan a holiday.

Psychologist Dr Cliff Arnall had his name next to the formula, which was as follows:

([W + (D-d)] x TQ)/M x NA

In the formula, W=weather, D=debt, d=monthly salary, T=time since Christmas, Q=time since failing our new year’s resolutions, M=low motivational levels, and Na=the feeling of a need to take action.

If that's confusing, don't worry - it barely makes any sense. Dr Arnall did not give any idea how to measure the values - what is one unit of "low motivation levels", for example?

This did not stop him producing a similar formula for Wall's Ice Cream in 2006 claiming to have discovered the happiest day of the year (it usually falls close to June 21).

Despite the fact it is clearly rubbish, the idea of Blue Monday has stuck in the public consciousness, and people are often unaware that it isn't real.

To his credit, Dr Arnall appears to have realised his error. His pinned tweet on Twitter includes the hashtag #StopBlueMonday.

Reassuringly, there is no scientific evidence to suggest today is any more miserable than any other given day of the year - although Seasonal Affective Disorder, tight budgets after Christmas and hating Mondays could have an effect.

So that's something to cheer you up - if you're feeling bad today, take heart in knowing you might not feel any better tomorrow.