I had following reasons to think so:
In the script there's no reference at all to Vito's birthday, instead we read:

MAMA
Please, do we have to talk about
the war at the table? On Christmas,
much less.
And later:

DON CORLEONE
I know. I know, Michael. It's
Christmas, your brothers and
sister
are all here -- we are happy.
Let's not spoil this.

In the movie this scene has been changed, I think Coppola at first meant it to take place at Christmas, but later made a wiser decision to retain original date - Puzo says in the novel that Mike enlisted on the day after Pearl Harbor, so he simply couldn't be there at Christmas.
So we have some lines added to tell us when it was:

SONNY
what do you think of the nerve those Japs - those slanty eyed bastards, huh. Dropping bombs in out back yard - and on Pops birthday you know!

FREDO
Now they didn't know it was Pop's birthday.

And later:
SONNY
Nice -- real nice -- break your fathers heart on his birthday.

And at the end of the scene they all congratulate Vito.