Blue Monday is a name given to a date in January stated, as part of a publicity campaign by Sky Travel, to be the most depressing day of the year. However, the whole concept is considered pseudoscience, with its formula derided by scientists as nonsense.
This date was published in a press release under the name of Cliff Arnall, at the time a tutor at the Centre for Lifelong Learning, a Further Education centre attached to Cardiff University. Guardian columnist Dr. Ben Goldacre reported that the press release was delivered substantially pre-written to a number of academics by public relations agency Porter Novelli, who offered them money to put their names to it. The Guardian later printed a statement from Cardiff University distancing themselves from Arnall: "Cardiff University has asked us to point out that Cliff Arnall... was a former part-time tutor at the university but left in February."
This date allegedly falls on the Monday of the last full week of January. The date was declared by Arnall to be 24 January in 2005, 23 January in 2006, 22 January in 2007, 21 January in 2008, 19 January in 2009 and 18 January in 2010. In 2011 there was confusion about the correct date; some claimed it to be 17 January 2011 while others stated Blue Monday was on 24 January 2011. In 2012, the most depressing day of the year was said to be 23 January. In 2013, Blue Monday was reported to be on 21 January,  and 20 January in 2014.
According to a press release by a mental health charity, the formula is:
where weather=W, debt=d, time since Christmas=T, time since failing our new year’s resolutions=Q, low motivational levels=M and the feeling of a need to take action=Na. 'D' is not defined in the release, nor are units.
Arnall says the date was calculated by using many factors, including: weather conditions, debt level (the difference between debt accumulated and our ability to pay), time since Christmas, time since failing our new year’s resolutions, low motivational levels and feeling of a need to take action. Writing about the calculation, Ben Goldacre stated: "... the fact is that Cliff Arnall's equations ... fail even to make mathematical sense on their own terms." Dean Burnett, a neuroscientist who has worked in the psychology department of Cardiff University, has described the formula as "farcical", with "nonsensical measurements".
Mr Arnall also says, in a press release commissioned by Wall's, that he has calculated the happiest day of the year—in 2005, 24 June, in 2006, 23 June, in 2008, 20 June in 2009, 19 June,  and in 2010, 18 June. So far, this date has fallen close to Midsummer in the Northern Hemisphere (June 21 to 24).