Complex sentence: The most notorious and the best documented massacre took place on 20--21 June 1940 when three transports with 358 inmates were sent from Pawiak to the place of execution at forest glade near Palmiry.
Split into simpler sentences: The most notorious and the best documented massacre took place on 20--21 June 1940. On that days three transports with 358 inmates were sent from Pawiak to the place of execution at forest glade near Palmiry.