These timeline anomalies are vexing, but I guess they're part of the freight that's carried in this immensely long trilogy, where moving from era to era, movie to movie, can cause inconsistencies:
--Vito's date are irrefutably chiseled in his tombstone.
--The Great Five Families Massacre occurred in 1955 shortly after Vito died. Note Clemenza's '55 Cadillac, which he drives (with a flowerbox containing his shotgun) to kill Stracchi in the elevator.
--Hyman Roth was born in 1891 because, as his Havana wedding cake shows, he's 67 in December 1958. (In a deleted scene from GFII that takes place around 1920, Clemenza recruits the teenage Roth, who looks to be about 10 years younger than Vito. Vito's age-appearance would have been right for a guy born in 1892, but Roth's wouldn't have been. FFC simply adjusted reality to make Roth a disciple of Vito. Perhaps the scene was deleted because, in the rest of GFII, Roth speaks of Vito as a partner and contemporary.)
--The Don was shot shortly before Xmas 1945. Sollozzo and McCluskey were shot either shortly before or after New Year's Day, 1946 (attest the "Happy New Year" banner hung in the rear of Louis Restaurant).
--The dates start to get less definitive after this. Many paisani place the subsequent action in the 1949-51 time frame because, in the scene where Sonny beats up Carlo, we see a tattered "Dewey for President" poster. Since Dewey ran for President in '48, 1949 or later seems logical for Connie's pregnancy, Sonny's beating of Carlo, Michael and Apollonia, etc. BUT: Dewey also ran for President in '44, so the poster could have been left over from that campaign. That would place the action in 1946, which is what I believe. Other evidence: If Connie were pregnant with Victor in '46, he'd have been 12 in '58, old enough to be "arrested for some petty theft..." We see GI's in a jeep in Sicily--right for Allied occupation in '46, wrong for '49 or later. In the scene where Michael woos Kay in New Hampshire, his car is a '47 Cad and we see a '47 Ford woodie station wagen in the background. These cars certainly could have been in use in '50 or '51, but FFC usually selected new or nearly new cars in the Trilogy. Yes, Anthony would have been (and looked) seven years old for his First Communion in '58. That could be because the Michael/Kay New Hampshire scene was set in '50 or '51--or because it was really set in '47, and Michael and Kay either decided not to have kids right away, or maybe had trouble conceiving.