Blue Monday is the third Monday in January.

Why is today the saddest day of the year?

A mathematical equation was used to establish Blue Monday on the third Monday of January

ISABEL MÉNDEZ MALAGA.

It's Monday, the middle of January and dull, wintry weather is forecast in many areas. If all these factors are getting you down, don't worry - you're not the only one today.

This Monday, 16 January, it is "officially" all right to be low in spirits; it's Blue Monday, the saddest day of the year.

But why has the third Monday in January earned itself this rather depressing title? Its origins date back to 2005, when a travel agency, Sky Travel, asked psychologist Cliff Arnall, a researcher at Cardiff University, to define the most depressing day of the year. And it wasn't just out of curiosity. They clearly had commercial reasons behind the enquiry: to sell more holidays.

Arnall's answer was the third Monday in January and to get there he used a formula incorporating factors such as average salaries, the weather at this time of year and frustration at failing to keep New Year's resolutions combined with post-Christmas blues.

This is the equation on which he based his theory: [W+(D-d)] x TQ/M x NA. The W is weather which is added to the result of D (debt) minus d (monthly salary) This is multiplied by T, the time since Christmas, and Q, time since failure of attempt to give something up, divided by M, low motivational level, and multiplied by NA, the need to take action.

So, if this Monday you're not feeling as bright as usual, according to the calendar it's only to be expected.