The sea change occurred in 1941 when the Germans were shooting Jews, Poles, Gypies, Russians and others on the Eastern Front. Sometimes it would be a firing squad working in shifts to gun down hundreds and sometimes it was a machine gun team, either of which were known as Einsatzkommando and attached to each German group that was invaing Russia. SS Commander Erich Von Dem Bach-Zelewski complained to SS Commander Heinrich Himmler that the shootings were taking a toll on Germans doing the shooting. He said the men were going crazy or becoming homocidal maniacs. (No concern expressed about the fact of mass murder being committed.)

Himmler took the complaint seriously. The SS explored other methods, including putting people in trunks and funneling the exhaust into the back as the truck drove. The very gruesome method was deemed inefficient. Skipping some minor steps, Zyklon B, a cyaninde-based pesticide, was introduced and the infamous showers and subsequent mass creamation was employed. Here is when the mass killing really took off in the worst way.

Source: http://www.holocaustresearchproject.org/einsatz/bach-zelweski.html

Ok, so at this moment is the majority opinion in scholarship of the Holocaust and Nazi Germany that at some point in the time frame between October and December 1941 Hitler made the decision to kill all the Jews of Europe.

This time frame is interesting because that was when the anti-Jewish policy of the Nazis radicalized in a lot of ways. The Jews of the Soviet Union had been targeted for killing by the Einsatzgruppen since the attack on the Soviet Union in July 1941 and sometime in August/September 41 the Einsatzgruppen started killing not only men but also women, children and the elderly, razing whole communities.

In October 41 we have the first systematic killings of Jews outside of the Soviet Union in Serbia where the Wehrmacht started killing male Jews as part of their anti-Partisan reprisal campaigns. Around the same time, construction starts on the first death camps where the "Euthanasia Experts" of the Third Reich are dispatched to the East in order to construct camps deigned solely for killing people. At least on of those camps, Kulmhof, starts operations in December 1941. However, the capacities of this camp point to the fact that it was intended to operate locally and not for the whole of Europe.

Another piece of evidence is that in early December 1941 the local HSSPF of Lithuania ordered the shooting of a considerable number of German Jews who had been deported there but was reproached for it by Himmler. Thus it seems like the decision to murder all the Jews of Europe must have taken place somewhere inside this timeframe of escalation, most likely in early December.

The big problem is that there is no written order for this but this seems to stem from the fact that the Nazis were keen on avoiding the "mistakes" they had made when ordering the Euthanasia program, i.e. assigning strict responsibilities and thus stifling initiative by lower tiered institutions. Aiming to avoid this failure, it seems like, there had been a talk between Hitler and Himmler at some point in early December leading to the SS informing other state institutions about these plans at the Wannsee Conference, first planed in early December also but then pushed back to January.

Other historians, such as Browning and Kerhsaw, place the order in October claiming that in October it was decided to kill all non-German European Jews and only in December, the decision was made to also include the German Jews. In the end, the singular moment is porbably impossible to date exactly but October to December 1941 is the timeframe that is supported by the evidence.

Sources:

• Christopher Browning: Fateful Months.

• Ian Kershaw: The Nazi Dictatorship. Problems and Perspectives of Interpretation, (London, 1985, 4th ed., 2000).

• Christian Gerlach: Krieg, Ernährung, Völkermord.