Something that is popular but soon becomes unfashionable or ceases to be novel.

An old proverb states A wonder lasts nine days, and then the puppy's eyes are open alluding to dogs which, like cats, are born blind. The public are reckoned to be blind initially, but then their eyes are opened.

Alternative: The Latin word novena is a term used in the Roman Catholic Church for a devotion lasting for nine consecutive days. Once completed, a new one may be instituted and the other forgotten.