Here's what dictates the entire timeline for me.

Vito is shot just before Christmas 1945. Sonny brings him home the following February, 1946. Vito has to rise from his sick bed to take command of the family after Sonny's murder. Now, if we push Sonny's murder into 1947, that means that Vito was in his sick bed for around 12 months. That seems implausible to me.

I believe that Sonny was murdered in late spring or early summer, 1946. The Don's meeting took place during the summer, 1946. But, as the novel states, it was to be almost a year before Vito could bring Michael back. So, Michael is back by summer or so, 1947. He meets up with Kay during fall/winter 1948 (leaves on the ground and winter coats; "I've been back a year; longer than that I think."). The massacre takes place in 1950/51.