Day of the Dupes
in French history, was November 11th, 1630, when Marie de Medicis and Gaston Duc d'Orléans 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.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894