I don’t where this idea came from, but Sonny Corleone (James Caan) went down in a hail of machine-gun bullets sometime during the warmer months of 1947 or ’48, when Michael Corleone (Al Pacino) was hiding out in Sicily. Michael returned to the U.S. in ’50. The murder of the heads of the five families (along with that of the turncoat Carlo) happened on the day of the baptism of Carlo and Connie’s daughter in 1951. Check with any Corleone family timeline — this, this, this or this — and they all say ’47 or ’48. For some reason this Sonny Corleone Wiki page claims 10.3.51, but it’s wrong.