While Hitler is speaking, he is handed a note mentioning that bad weather will ground his plane. It is not explained in the movie, but Hitler cut short his speaking engagement (and avoided Elser's bomb) so that he could catch a train instead.
Based on historical events, but many elements from Elser's private life are fictitious.
During the interrogation Nebe admonishes Elser for detonating the bomb on November 8 but no explanation for the significance of this date was mentioned. November 8 was the anniversary of Hitler's Beer Hall Putsch, a failed coup attempt in 1923. Hitler famously returned each year to speak and commemorate the date, which made Elser's assassination plan possible.
During the interrogation of Elser by the German scientists, he is injected with Pervitin, which was the early version of what is known nowadays as crystal meth.
Not only was it the anniversary of the famous 'Beerhall Putsch', the venue was the Munich Beer Hall where Hitler returned each year to speak and commemorate the date.