The legend of how “Christmas in July” came to be is a bit of a mystery ― some stories say that it was invented at a summer camp in the 1930s. Regardless, the first known pop culture reference was the 1940 movie Christmas in July. “The plot? A man’s colleagues prank him into thinking his work won a $25,000 prize—and he goes on a jolly spree of generosity”. That said, the off-season celebration has become a marketing and retail phenomenon.