In Chapter 5, the camp was to be evacuated because the Russians were approaching. Those who stayed behind in the hospital "were quite simply liberated by the Russians two days after the evacuation".

Why did they evacuate Buna?

Eliezer understands all too well that the Nazis wouldn't think twice about killing every last patient in the infirmary, whatever their physical condition. So the decision is made to leave, and as Eliezer doesn't want to be separated from his father, he is coming with him.

What happened in Buna in the book night?

Buna: Also known as Auschwitz III, Buna is a smaller work camp about a four-hour walk west of Auschwitz. Eliezer and his father are sent to Buna to work in a warehouse sorting electrical parts. ... Only 12 of the 100 men in Eliezer's car survive. Eliezer and about 20,000 prisoners are liberated on April 11, 1945.