“Writer Olga Khazan speculates that perhaps the answer lies with the 1946 film It’s a Wonderful Life—with its famous scene in which Jimmy Stewart’s character George Bailey contemplates suicide—and the way in “which basic-cable networks put [it] on heavy rotation as Christmas nears.” I wondered, however, whether psychiatrists and clinical psychologists might also have played a role in associating the holidays with acute mental health problems.” http://www.psychiatrictimes.com/major-depressive-disorder/holiday-syndrome-who-exactly-came-idea-those-christmas-blue
The author of this article gets a bit esoteric and technical . . . as one might expect from a writer for a psychiatric publication.
"It was James P. Cattell9 (d. 1994), a Harvard- and Columbia-trained psychiatrist and psychoanalyst, who seems to have been the one to have shifted the discussion in a new direction. In 1955, he coined the term “the holiday syndrome,” describing it as a reaction in some patients that manifests itself beginning around Thanksgiving and ending a few days after January 1st. It is characterized, he insisted,
'. . . by the presence of diffuse anxiety, numerous regressive phenomena including marked feelings of helplessness, possessiveness, and increased irritability, nostalgic or bitter rumination about holiday experiences of youth, depressive affect, and a wish for magical resolution of problems.9p39"