Majdanek (in Lublin, Poland) was the first major camp to be liberated. On 22 July 1944, after a sudden advance, Soviet forces reached the camp. They found the gas chambers more or less intact. Some camps in Poland were dissolved by the Nazis as the Soviets approached. The liberation of most camps took place in the first few months of 1945.
The camp at Breendonk, near Antwerp in Belgium was liberated by the Allies early in September 1944. they weren't all liberated at once they all had different dates, but Auschwitz got liberated on January 27, 1945 by the Soviet Army.