nine-day wonder

From nine +‎ day +‎ wonder (“something that causes amazement or awe”). References to a period of nine days or nights to describe the length of a short-lived fad date from as early as the 14th century; see, for instance, Troilus and Criseyde (c. 1380s) by the English poet Geoffrey Chaucer (c. 1340s – 1400; spelling modernized): “Ek [besides] wonder last but nine days never in town”.