In modern lexicon, a “flavor of the month” is something that’s popular, but only for a short amount of time.
The oldest known use of the phrase comes from a 1930s ice cream advertisement from Sealtest Dairy. Competing ice cream companies often used the phrase to highlight certain monthly offerings and attract more customers.
“Flavor of the month” really took off in the 1940s, when other industries adopted it. By 1947, it was describing everything from relationships to fashion trends.