sprat day, n. — Green’s Dictionary of Slang

sprat day n.

[the traditional arrival of sprats in the market at approx. the same time, i.e. early November each year; November 9 is the day on which London’s Lord Mayor takes office]

the Lord Mayor’s Day in London.

1861

H. Mayhew London Labour and London Poor I 69/2: Sprats [...] are generally introduced about the 9th of November. Indeed ‘Lord Mayor’s day’ is sometimes called ‘sprat day’.