Blue Monday is a name given to a day in January (typically the third Monday of the month) reported to be the most depressing day of the year for countries in the Northern Hemisphere. The concept was first publicised as part of a 2005 press release from holiday company Sky Travel, which claimed to have calculated the date using an equation. (Wikipedia)
It is calculated using a series of factors in a (not particularly scientific) mathematical formula. The factors are: the weather, debt level (specifically, the difference between debt and our ability to pay), the amount of time since Christmas, time since failing our new year’s resolutions, low motivational levels and the feeling of a need to take charge of the situation.