Blue Monday
Blue Monday is held on January 18. 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
Cable Car Day on January 17 (In early 1871 British-born, US resident Andrew Smith Hallidie received the first patent in relation to the cable car);
Ditch New Year's Resolutions Day on January 17
Thesaurus Day on January 18 (It is celebrated every year on January 18th, which is the birthday of the author of Roget's Thesaurus, Peter Roget. Peter Roget was born on January 18th in 1779);
Winnie the Pooh Day on January 18 (This day was created to celebrate the birth of A.A .Milne in 1882);
World Snowman Day on January 18
Tin Can Day on January 19 (Peter Durand patented the tin can in 1810);
Camcorder Day on January 20 (On this date in 1982, the camcorder was first introduced into the public marketplace by Sony Electronics)