Evil May Day
(1517). So called because of the riots made on that day by the London apprentices, who fell on the French residents. The ringleaders, with fifteen others, were hanged; and four hundred more of the rioters were carried to Westminster with halters round their necks, but were pardoned by “Bluff Harry the King.” The Constable of the Tower discharged his cannon on the mob assembled in tumult in Cheapside Way.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894