Wikipedia
Montagsloch
The so-called Montagsloch in Essen was a building project during the time of National Socialism in Germany. It should have become one of the biggest stadiums in Germany.
Its name probably refers to the day when 35 Russian forced labourer and maybe also some other people were killed by the Gestapo of Essen at this place. It was on March 12, 1945, a Monday (in German:"Montag").
Some of the murderers were judged to long term prison sentences by a British military court in 1948.