Short answers, pulled from the story.
The Massacres of Poles in Volhynia and Eastern Galicia resulted from volatile demographic realities, repeated Polish state policies shifting between assimilation and repression, and radical nationalist violence by the Organization of Ukrainian Nationalists. Harsh policies implemented by the Second Polish Republic often resulted from OUN-B violence but contributed to further deterioration of relations.
On the 9th of February 1943, a UPA group commanded by Hryhory Perehyniak assaulted the Parośle settlement in Sarny county, which is recognized as the first mass murder committed by the UPA in the area. The largest wave of attacks took place during July and August 1943, with assaults continuing until spring 1944 when the Red Army arrived in Volhynia.
The Institute of National Remembrance estimates that 100,000 Poles were killed by Ukrainian nationalists, with 40,000 to 60,000 victims in Volhynia and 30,000 to 40,000 in Eastern Galicia. Other historians provide varying figures ranging between 60,000 and 80,000 total Polish victims across both regions.
Major massacres occurred on the night of the 22nd of April 1943 in Janowa Dolina where 600 people were killed, and on the 11th of July 1943 when UPA units attacked 96 Polish villages. The most infamous massacre took place on the 28th of February 1944 in Huta Pieniacka involving over 1,000 inhabitants.
In 2008 Poland's Parliament adopted a resolution calling UPA crimes against Poles crimes bearing hallmarks genocide. In 2013 it adopted a resolution calling them ethnic cleansing with hallmarks genocide, and on the 22nd of July 2016 Poland's Sejm established the 11th of July as National Day Remembrance for Victims Genocide committed by Ukrainian nationalists citizens of the Second Polish Republic.