Green’s Dictionary of Slang

keeping the passover phr.

[orig. known as hold a Rag Fair or hold a Monmouth Street. The association of Jewish old-clothes sellers with Monmouth Street led to the introduction of Passover, the Jewish spring festival commemorating the Exodus from Egypt; presumably + added ref. to the ‘passing over’ of the fresh air]

[late 19C] (W.I.) spreading out one’s clothes for an airing.