Why didn't Elie fast on Yom Kippur?
In the fifth chapter of Elie Wiesel's Night, the concentration camp inmates are debating whether they ought to participate in fasting for Yom Kippur since it would be physically dangerous to deprive themselves of calories even further than they already are deprived in their daily minuscule rations of food. Some of the prisoners say that the danger of fasting is actually an important reason to go ahead and do it. It would prove that their devotion to God is more powerful than the suffering they are going through.
However, Eliezer has begun to have some doubts about God recently, and he decides not to fast for two reasons: first, his father has forbidden him to fast (presumably because of the starvation danger, since Eliezer is still young and has a father who wants to keep him safe), and second, Eliezer wishes to rebel against doing an act that would please God since, as he says in chapter five, "I no longer accepted God's silence". In this way he protests the fact that the God he has always believed in is not fulfilling Eliezer's expectation that God should protect his chosen people from the torture and death they are enduring in the camps.
It is also noteworthy that Eliezer says that after swallowing his soup and nibbling his bread, he "felt a great void opening". This indicates that his act of rebellion against God has been uncomfortable for the once-pious boy who had been training to become a rabbi in his village before the Nazis came. Rejecting something that had been such a huge part of his previous phase of life, Eliezer is left for the present with nothing else to take its place.