It was one of those directorial license anomalies that we've discussed many times:

FFC had to put Michael in Havana on New Year's Eve, in order to provide the historical authenticity of Batista abdicating and Castro taking over. Since events had to move fast because his and his families' lives were threatened, FFC had to move Anthony's First Communion to mid- or late-December, then have him leave Tahoe immediately after the shooting--to Miami, to New York (hence the snow) then to Havana. Perhaps he got a dispensation from the Church to move Anthony's communion to December.

And, while he was at it, he must have gotten another dispensation on Tahoe weather--daily highs are around 40 in mid-December, lows about 18. But nobody's wearing cold weather gear at Anthony's party.