During the final scene flashback, they are talking about the attack on Pearl Harbor happening on "Pop's birthday". It is also revealed that this led Michael to enlist in the army "this morning". Since the attack happened in Pearl Harbor at 8AM Hawaiian time, news of the attack couldn't have reached New York before 1PM. See more
The godfather was born Vito Andolini, in the town of Corleone in Sicily. In 1901 his father was murdered for an insult to the local Mafia chieftain. His older brother Paolo swore revenge and disappeared into the hills, leaving Vito, the only male heir, to stand with his mother at the funeral. He was nine years old.
[gunshots and screams
[subtitled from Italian
They've killed the boy! They've killed young Paolo! They've killed your son Paolo!