Because of a lack of sunshine, post-Christmas debt and failed New Year's resolutions, the third Monday in January, called Blue Monday, is said to be the gloomiest day of the year.
Blue Monday is calculated using various factors in a (not particularly scientific) mathematical formula. The factors are: weather, debt level (specifically, the difference between debt and one's ability to pay), amount of time since Christmas, time since failing one's New Year’s resolutions, low motivational levels, and a need to take charge of one's situation. These factors come together in a perfect storm for what is believed to be the most depressing day of the year – Blue Monday.
Blue Monday actually helps shine a light on depression and mental health.