The Holocaust was the result of a very gradual progression, over centuries, of anti-Semitism (hatred of the Jewish people). This progression resulted in the desire to eliminate the Jewish people, from predominantly Christian lands, entirely. What we usually refer to as the Holocaust - the systematic eradication of the Jewish people from Europe - entered the height of implementation in 1941 following the German invasion of the Societ Union. However, the events leading to the implementation of this systematic plan to kill all the Jewish people, made it possible. For example, during the 1930s as the Nazis rose to power they introduced many anti-Jewish decrees that set the tone for increased Jewish persecution. So - pinpointing an exact moment when the WWII Holocaust began is almost impossible.
Pinpointing the start of the Holocaust is open to debate. I think a good place to begin would be with the passage of the Nuremberg Laws in September 1935. These laws denied the Jewish people the rights of citizenship. Jews couldn't marry Germans. By the end of 1935, Jews could not vote or hold public office. By 1936, several professions (such as teaching and journalism) were no longer open to the Jews. By 1938, Jews couldn't practice law or medicine. Another turning point was the night of November 9, 1938. This is known as Kristallnacht, or the Night of the Broken Glass. On that night, there was a massive destruction of Jewish property, homes, and synagogues. The Gestapo terrorized the Jews to make it look like the uprising was spontaneous. Many Jews died and were hurt. The next day, many Jews were arrested. They were freed only when they agreed to leave Germany and surrender their possessions. Then the concentration camps and extermination camps were set up. Jews were deported to these camps. At the Wannsee Conference in 1942, the Final Solution was developed. The Jews were to be exterminated as well as other groups of people such as Gypsies and homosexuals. By the time the war ended in Europe, the Holocaust wiped out about 11-12 million lives. Six million were Jewish lives. The Holocaust was a devastating event for many people in Europe.