E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Day of the Dupes,
in French history, was November 11th, 1630, when Marie de Medicis and Gaston Duc dOrléans extorted from Louis XIII. a promise that he would dismiss his Minister, the Cardinal Richelieu. The cardinal went in all speed to Versailles, the king repented, and Richelieu became more powerful than ever. Marie de Medicis and Gaston were the dupes who had to pay dearly for their short triumph.