Yesterday was the 79th anniversary of the Black Sunday (the...

Yesterday was the 79th anniversary of the Black Sunday (the worst day) of the Wolyn massacres:

https://ipn.gov.pl/en/news/9698,The-Deputy-President-of-the-IPN-paid-tribute-to-the-victims-of-the-Volhynia-Mass.html

The massacres in Volhynia were anti-Polish genocidal ethnic cleansings 1943-45.
The massacres took place within Poland’s borders as of the outbreak of World War II, not only in Volhynia,
but also in other areas with a mixed Polish-Ukrainian population, especially the Lwów, Tarnopol, and Stanisławów voivodeships.

The perpetrators were the Organization of Ukrainian Nationalists−the Bandera fraction (OUN-B)
and its military wing, called the Ukrainian Insurgent Army (UPA)

It has to be noted that a number of Ukrainian peasants joined in the killing of the Poles.