In Germany we call it "Bromberger Blutsonntag" and it happened!
Here it is in English: URL
It is hard to say how many Germans died exclusively during the Bloody Sunday. German author Peter Aurich gave a number of German civilian deaths in Bydgoszcz as 366 deaths., another German historian, Hugo Rasmus, estimates the number of ethnic German deaths in Bydgoszcz as "at least 415". Two Polish historians - Włodzimierz Jastrzębski and Czesław Madajczyk against the Polish soldiers in the city, estimate ethnic German deaths respectively 103 (Jastrzębski), and about 300 (150 on September 3, the rest in the days after) victims (Madajczyk).
It looks like there was a massacre of the German minority in town of Bydgoszcz two days after the German invasion of Poland.
I've never heard about it, have you?
I have always been confused about the historical reality of what happened in the Danzig corridor in 1939, which many claim was the direct provocation for the German attack on Poland...
The massacre in Bydgoszcz accrued two days AFTER the start of the invasion.