The Godfather Part II start in the fall of 1958 according to the script. It must have been an exceptionally warm October at Lake Tahoe because you see some people swimming. Filming the Lake Tahoe party started in early October as well, and unlike in the movie, the actors had to wear thermal underwear because of the cold.

According to Coppola's notes a young Vito returned to Sicily in 1922 (the movie trailer says 1921).

I've also seen some pages of the recently released The Godfather Notebook and Coppola indeed states that Sonny's death is in 1946. However, he also states that Michael returns in 1950 and that the massacre is in 1955, which we already knew based on Vito's tombstone.