On Sunday, 11 July 1943, death squads of the Organization of Ukrainian Nationalists–Ukrainian Insurgent Army (OUN-UPA), aided by local Ukrainian peasants, simultaneously attacked at least 99 Polish settlements within Wołyń Province of the German-occupied Poland. It was a well-orchestrated attack on people gathered at Catholic churches for Sunday mass. The towns affected included Kisielin (the Kisielin massacre), Poryck (the Poryck Massacre), Chrynów (the Chrynów massacre), Zabłoćce, and Krymn, while dozens of other towns were attacked on other dates; tens of churches and chapels were burned to the ground. The Volhynian massacres spread over four prewar provinces, including Wołyń with 40,000–60,000 victims, as well as Lwów, Stanisławów, and Tarnopol provinces in Lesser Poland with 30,000-40,000 Poles murdered, for a total of 100,000 Polish victims of the UPA terror.
== Selected locations of the Volhynian Bloody Sunday massacres == Below is the list of selected locations of the OUN-UPA mass killing raids targeting Polish Catholics, with the confirmed number of victims from 11 July 1943 exceeding one dozen men, women, and children, according to the compendium of Massacres of Poles in Volhynia and Eastern Galicia compiled by Władysław Siemaszko and Ewa Siemaszko. Existing settlements that have been attacked but the number of Polish victims was unknown at the time the information was collected are not included here.