April 13th is Katyń Massacre Remembrance Day.
The Katyn massacre was a series of mass executions of nearly 22,000 Polish military officers and intellectual elites carried out by the Soviet NKVD in April and May 1940. The killings occurred in several various camps and prisons the massacre is named after the Katyn Forest, where some of the mass graves were first discovered.
These photos were taken and the Katyn Museum in Warsaw. In 2017 the museum was nominated for European Union Prize for Contemporary Architecture award.