Those who took part in the coup d'état which effected
the fall of Robespierre, with the desire of restoring the legitimate
monarchy. So called because the Reign of Terror was brought to an end
on the ninth Thermidor of the second Republican year (July 27th, 1794).
Thermidor or “Hot Month” was from July 19th to August 18th. (
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894