This was the massacre of approximately 8000 Norman French by Sicilians on March 30, Easter Monday, 1282.

It began at Palermo, as the bell was tolling for evening service. The Sicilians had already been involved in furthering their designs against Charles of Anjou, because the Normans, since their occupation of Sicily, had been utter bastards. One of them, a man named Drochet, said something inappropriate to a Sicilian bride. A young Sicilian stabbed Drochet with his sword, and in the ensuing melee, the Sicilians killed ~200 Normans then ran through the street yelling "Let the French die," and went full-out ballistic, killing any Norman they saw.

You may be interested in the book The Sicilian Vespers : A History of the Mediterranean World in the Later Thirteenth Century, by Steven Runciman.