Katyn Wood is an event in Polish history. The mass graves of a number of army officers, government officials, & other intelligensia were found in Katyn Wood. The Soviet Union claimed for many years that these people were killed by Nazi Germany; the Nazis claimed that the Soviets killed them. Communist Poland refused to discuss the event, & I have no idea if the post-Communist government has made an official statement concerning the event. I have not seen it referred to as a "massacre", though.
You haven't looked it up in Wikipedia, then.