According to Wikipedia .....

“Blue Monday is a name given to a day in January claimed to be the most depressing day of the year. 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. It takes into account weather conditions and this only applies to the Northern Hemisphere.

The idea is considered pseudoscience, with its formula derided by scientists as nonsensical.”