As far as the West was concerned, on 15 April 1945.
That’s the day British and Canadian troops liberated the Bergen-Belsen concentration camp. They found about 13,000 human corpses lying around unburied, and about 60,000 starving and sick prisoners, who were dying at a rate of 500 per day.
Bergen-Belsen was not even the worst of the German camps — it was ‘only’ a concentration camp, not a death camp. However, it was the first major camp to be discovered by Western Allied troops, and photographs of the horrors found there were broadcast all around the world.
Similarly, there had been plenty of stories and rumours circulating of how the Nazis treated their victims — but until April 1945, many people had dismissed them as just that, merely rumours. After all, there had been plenty of horror stories told about the Germans during the First World War — of corpse factories, crucified Canadians, and mutilated nuns.
When that war ended in 1918, it was discovered that many of those stories had been exaggerated or invented. People who considered themselves to be sensible and balanced therefore took the lurid stories about new Nazi atrocities with a certain amount of scepticism — until the photographic proof from Belsen established that the accounts of Nazi horrors had been underestimated, not exaggerated.
The industrial-scale, entirely deliberate and cold-blooded slaughter under Hitler surpassed anything which had previously been known. There had been massacres before, but never on this scale, and never implemented with the same soulless efficiency.