how do the prisoners celebrate Rosh Hashanah and Yom Kipper?

The Jews inside Buna come together for a service to celebrate Rosh Hashanah.

Eliezer wonders, angrily, where God is and refuses to bless God’s name because of all of the death and suffering He has allowed. Eliezer thinks that man is strong, stronger than God. During this year’s Rosh Hashanah, unlike all previous years, Eliezer is not asking forgiveness for his sins. Rather, Eliezer feels himself to be "the accuser, God the accused."

Also Some of them fasted;some thought that was a quick way to death, so they did not; sang to God to praise him. For Rosh Hashanah they prayed together while they before they ate their soup. They were in the Appelplatz while they prayed.