I found yet another detail in the first sentence of chapter 17.

"The war of 1947 between the Corleone Family and the Five Families combined against them proved to be expensive for both sides."

This confirms Sonny's death in 1947 as he is killed in chapter 19.

And the other confirmation can be found in the first sentence in chapter 22.

"Lucy Mancini, a year after Sonny's death, still missed him terribly, grieved for him more fiercely than any lover in any romance."

Later on in the same chapter it is mentioned that it's 1948.

And as chapter 19 implies that Sonny was killed in the summer it's therefore most likely he died in the summer of 1947.

Shortly after, the meeting of the dons takes place in chapter 20 and the first sentence of chapter 21 implies that Michael returned in 1948.

"But it was to be nearly another year before Don Corleone could arrange for his son Michael to be smuggled back into the United States."

So Michael probably returned in the spring of 1948. But then again, it could also be 1947 as TB's notes point out.