Isobars are atoms that have same atomic mass but different atomic number. They have different atomic numbers because they have different numbers of protons. They have the same atomic mass because they have just enough neutrons to make the same total of nucleons. Isobars occur at different places in the Periodic Table.
The Holocaust (also called Ha-Shoah in Hebrew) refers to the period from January 30, 1933 - when Adolf Hitler became chancellor of Germany - to May 8, 1945, when the war in Europe officially ended.
There are many answers to that question. A lot of them have to do with events in the Second World War. Some will date the beginning of the Holocaust to September 1939, when the Germans attacked Poland; others to the invasion of the Soviet Union in June 1941; still others to the Wannsee Conference of January 1942.