Flavor of the month
Something that is prominent in the public eye for a short time then fades out of interest. Originally a term of approval for something that was up to the minute and desirable. It has been used ironically from the late 20th century to pass disdainful comment on things which pass out of fashion quickly; for example, the "one hit wonders" of the music business.
This website is based in England and so we prefer UK spellings. In this case "flavor" is more appropriate than the UK "flavour", for reasons that will become apparent.
The phrase dates back to 1930s USA and originated in the advertising slogans of ice-cream companies there. It isn't absolutely clear which company first used this in their campaigns. Search as I may for references to this phrase I can't find any that pre-date 1936. The earliest I've found is an advert in The Mansfield News Journal, June 1936:
"If you haven't tried Sealtest Fresh Strawberry Ice Cream, made by Telling's, you're missing a real treat. It's the flavor-of-the month for June, selected by the Sealtest Jury."
The idea must have taken off quite quickly. There are several other US ice-cream companies that began using the term in the 1940s. By 1946 it had become well enough established for a trade association to have taken an interest in it. The September 1946 issue of the Ice Cream Review records:
"The Illinois Association of Ice Cream Manufacturers has set up a committee which will give serious study to a suggested flavor and flavor-of-the-month program for 1947."
That 'program' and 'serious study' suggests that this phrase was well-entrenched by 1946.
See other phrases that were coined in the USA.