Allied governments certainly knew something of the Holocaust by the war's later stages, though it's doubtful that they fully grasped the enormity of what was happening, the worst of it in the east. Some reports were disbelieved as unthinkable. Prior Allied awareness of Nazi atrocities - and readiness to document them - and shock at what they found are quite compatible. They knew, but they didn't know - or sometimes couldn't bring themselves to believe - the worst.

US soldiers liberated Buchenwald on April 11, 1945, and Eisenhower was very meticulous in its documentation and publicity. It was very heavily covered by the American press. So the New York Times ran articles in April 1945 with headlines like:

April 13: "PRISON CAMP IS OVERRUN — 20,000 Political Captives Are Found at Buchenwald"

April 23: "BUCHENWALD TOUR SHOCKING TO M.P.'S — Congressmen also stunned — British Group Hears Details of Horrors at Oscwicim"

April 30: "BUCHENWALD WORSE THAN BATTLEFIELD - Its Skinny Corpses, Seen in Camp Tour, Emphasize Cost of Human Freedom's Loss - THEIR DEATHS HORRIBLE - New Details of Atrocities Told"

Other newspapers referred to it in the headlines as a "DEATH FACTORY" and talked about the atrocities and other things. So this news spread very quickly, and more and more information came out about it, by the end of April.

Technically these are just prior to the end of the war in Europe, but were very close to it. This is a separate issue from whether the Allies knew much about the camps or their purpose.

Both are true, albeit the difference is largely semantics. Troops began liberating concentration camps as they pushed into German territory, which was near the end of the war, but the full extent of the Holocaust and it's almost industrial extermination of people's wasn't pieced together in full until after the war - which included interrogating Germans who participated in it