It's quite possible Vito was shot on December 21, 1945. That day was a Friday. Vito says to the guy who's helping him with his coat, "Merry Christmas, Dano." Since Xmas fell on the following Tuesday, perhaps the salutation indicated that Genco Pura Olive Oil Co. would be closed until after Xmas.

Michael got to the Mall that night. Tom was kidnapped and released that night. BUT:

Since none of the other families were involved yet, we can assume it was still between Tattaglia/Sollozzo and the Corleones. Bruno Tattaglia was killed very soon after Vito was shot. But Sonny said that Sollozzo wanted a meeting, and that Bruno's killing canceled out Vito's shooting. So, in effect, Sollozzo was proposing a truce between Tatt/Sol and the Corleones until the meeting.

As for the restaurant: We did see a "Happy New Year" decoration in the back. It could have been in anticipation of 1/1/46 (if the meeting took place before 1/1), or it could have been left over (if the meeting took place after 1/1). I strongly tend to believe the former. I doubt that anyone would have wanted the final accounting to be delayed until after the New Year (as in Michael advising Sonny, "You can't wait..."). The absence of visible Xmas decorations in the restaurant could mean that the meeting happened after Xmas but before New Years, by which time they might have taken down the decorations. Or it might mean that the camera angle didn't pick up Xmas decorations.