Well, you've hit on one of the Big Anomalies of GFII:

Yes, it appears to be a warm day (or at least not cold). And, it would fit the Church's schedule for First Communions, which I think are traditionally held on Pentecost Sunday, seven Sundays after Easter. That would put Anthony's party sometime in June, But, the shooting happened that evening, and Michael left immediately to see Roth in Miami. Then he visited Pentangeli in NY--and it was snowing. Then he went to Havana, and was there at New Year's Eve. He obviously didn't stretch all this out over six or seen months, so Anthony's First Communion and the shooting had to have happened sometime in December.

So, why was Johnny Ola wearing an overcoat? My guess is, to keep warm. I think he told Coppola: "Listen, Francis, I don't care what you say, it's cold as hell in Tahoe in December, and I don't have a mink stole to keep me warm."