From Patron Saint Index:
Arrested with several of his brothers on 19 September 1939 following the Nazi invasion of Poland. Others at the monastery were briefly exiled, but the prisoners were released on 8 December 1939, and the men returned to their work. Back at Niepokalanow he continued his priestly ministry, The brothers housed 3,000 Polish refugees, two-thirds of whom were Jewish, and continued their publication work, including materials considered anti-Nazi. For this work the presses were shut down, the congregation suppressed, the brothers dispersed, and Maximilian was imprisoned in Pawiak prison, Warsaw, Poland on 17 February 1941.
On 28 May 1941 he was transferred to Auschwitz and branded as prisoner 16670. He was assigned to a special work group staffed by priests and supervised by especially vicious and abusive guards. His calm dedication to the faith brought him the worst jobs available, and more beatings than anyone else. At one point he was beaten, lashed, and left for dead. The prisoners managed to smuggle him into the camp hospital where he spent his recovery time hearing confessions. When he returned to the camp, Maximilian ministered to other prisoners, including conducting Mass and delivering communion using smuggled bread and wine.
In July 1941 there was an escape from the camp. Camp protocol, designed to make the prisoners guard each other, required that ten men be slaughtered in retribution for each escaped prisoner. Francis Gajowniczek, a married man with young children was chosen to die for the escape. Maximilian volunteered to take his place, and died as he had always wished - in service.
More from Catholic Culture
Saint of the Day
Esther, A Catholic Mom in Hawaii has lots of information on this important saint.