Yes, of course, you are right, it would be anyway in December and that means Christmas tree, shopping and mood.
What I tried to say was how I see the possible genesis and development of the idea of this flashback.
They were going to show Michael with his son, so they wished to remind us the difficulties his own father had with him. The best possible moment to choose was the ultimate point of Michael’s defying “his fathers express command” and joining the army. They intended to show the reaction of all the family, so the whole family had to be gathered at the same place, which meant some family celebration, but no strangers, something closely private.
It would also show, as a contrast, the gaiety of old good days, and the people he loved, most of them gone, and as an additional thrilling point – almost half of them killed by that very college boy, who was having his own plans for future, and never let anyone bend his will even then.
Coppola chose Christmas day, Dec.25, as most eligible for those purposes date in December, as Mike enlisted in Dec., after P.H. It was written in the script as Christmas, and it atoned well for bringing Tessio, another victim of Michael’s, into the scene, because Christmas is a day that we usually spend with our family, and he had no family, unlike Clemenza and Genco, so it is possible that they invited him so that he wouldn’t sit alone. Also Carlo could be invited- we invite the friends of our children for Christmas, nothing’s strange here.
But there was a big hole in this plan, and I guess it might be Puzo himself who pointed out the mistake to FFC. Michael enlisted not just “after P.H.”, but on the day after, so it would be impossible for him to be still there, and still less possible that he could keep his enlisting a secret and announce it more than 2 weeks later. Also would be strange to introduce the “Nervy Japs” topic so late, when it had to be discussed many times already. But it was necessary, because it gave Mike possibility to tell about his decision.
So they decided to make it Dec.7. As there was nothing to celebrate, they made this day Vito’s birthday (I hope with Puzo’s permission, or maybe it was his own idea). It was good move – It made Michael’s step more significant, and the presence of Tessio at the celebration, as one of Vito’s closest friends, more justified. But then, there had to be others - Clemenza and Genco. But they found their way out: we don’t see here, as in the script, everyone gathered and celebration begun. Maybe they would come in a minute later, or even came in together with Vito. Of course there was no Brando, so we couldn’t see Vito’s reaction or his talk with Michael, but that is still better. They were not necessary, cuz we can guess that well
In this version the presence of Carlo becomes puzzling, but well – everything cannot be perfect after all . I wonder one more thing: When did Mike’s holidays at college begin? Or he left for a day to attend his dad’s birthday?
See how terribly nitpicky I am!