Janusz Korczak was the pen name of Henryk Goldszmit, born on 22 July 1878 to an assimilated Jewish family in Warsaw, Poland. Korczak studied medicine, was a writer, and was the director of the Jewish orphanage in the city, which he continued to manage in the Warsaw ghetto.
Holocaust survivor Michael Zylberberg describes a special encounter with Korczak in his memoirs.
"A few weeks before Rosh Hashana  Janusz Korczak visited us at home with a new idea. He wanted me to help him organize services at the orphanage.
…He saw my surprised expression and, not waiting for questions, said, "At this particular time it is important to hold services at the orphanage. The prayers may give people a spiritual uplift in these tragic times.
…Within a few days everything was arranged. I found a good cantor…the children themselves got the hall ready…they laid down carpets and decorated the place with flowers smuggled from outside the ghetto. An ark and embroidered cover were obtained, and two scrolls.
…He [Korczak] would spend all his time in the hall among the children, standing in a corner will back from the front rows with a Polish prayer book in his hand, incongruously garbed in an old grey overcoat, army boots and a silk skullcap. He was deep in prayer.
The cantor, who had suffered a great deal and was a man of learning, put all his heart into his performance. His supplications sprang from personal experience, and never had an audience been carried away as this one. No one stirred. Even the children were glued to their seats…
Michael Zylberberg, A Warsaw Diary (London: Vallentine, Mitchell & Co. Ltd, 1969) pp 44-45
“The Telz ghetto was in the very worst part of the city. Most of the people lived in dilapidated houses, without windows, in cowsheds and stables, in dampness, cold and dirt. Most of the inhabitants of the ghetto were women [because the men had already been deported]. Rosh Hashanah came. The women gathered in the old synagogue, which was located in this part of the city, for the holiday service. There were hardly any machzorim or siddurim, nor was there anyone to serve as the chazzan. They all waited… Suddenly a sweet voice was heard: “Bless the Lord who is blessed,” and the congregation responded: “Blessed be the Lord who is blessed forever and ever” [the opening words of public prayer]. In front of the ark stood a young girl who prayed by heart, passage after passage, […] with the appropriate melody […] like a real cantor, and the congregation was swept after her. The girl also blew the shofar [ram's horn, blown on Rosh Hashanah to inspire prayers of forgiveness] She put her hands to her mouth like a shofar and emitted shofar-like sounds. “Shevarim [disconnected sounds]…” “Teru’ah [tremulous sounds]”… like a perfect shofar-blower who was blowing a real shofar. She also read the portion of the week like a true ba'al tefillah."
Chassia Gering-Goldberg, The Book of Telz: A Memorial to a Holy Community (Heb.) (Telz Survivors’ Organization in Israel).
Holocaust survivor Avraham Shdeour grew up in Czechoslovakia and was deported to Auschwitz at the age of 14. In his testimony, he describes the following event:
My father pulled me into a group of men who were praying, and I suddenly realized that I had forgotten how to pray, that the words of the Shema Yisrael had escaped me. I, who had known all the tefillot by heart since I was a young boy – mincha, ma'ariv [evening prayer], shacharit and also the tefillot of Shabbat – suddenly could not recall a word. I felt terrible. I said to my father, "I want to daven [pray], but I cannot, I can't remember the tefillot." He tried to recite them with me, and suggested that I repeat after him, but I felt I couldn't. I repeated, "Father, I want to daven," and he answered, "Daven however you can, a tefillah from the heart, a tefillah that you feel you can say"...
From the testimony of Avraham Shdeour, YVA 0.3/4503