Blue Monday
Blue Monday is held on January 15. Date for 2024. The third Monday is claimed to be the most depressing day of the year. This event in the second decade of the month January is annual. 
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The name was reportedly coined by psychologist Cliff Arnall in 2004, it falls on the third Monday in January every year. This has been disputed by others over the years. He came up with it after a holiday company asked him for a "scientific formula" for the January blues (the sadness that comes with this period).
In general Blue Monday is considered part of pseudoscience. All in all, it is considered fake and scientists don’t accept it as real.
Cliff Arnall talked about Blue Monday in the Independent in 2018. Many companies adopted the idea of Blue Monday and created campaigns based on its concept.
Similar holidays and events, festivals and interesting facts
Clean Off Your Desk Day on January 14
Dress Up Your Pet Day on January 14
Feast of the Ass on January 14
Organize Your Home Day on January 14
Hat Day on January 15
Appreciate A Dragon Day on January 16
Nothing Day on January 16 (This day was first practiced way back in 1973 by newspaperman Harold Pullman Coffin. The goal of the day is "to provide Americans with one national day where they can just sit without celebrating, observing, or honoring anything.")