The Japanese attack on Pearl Harbor took place about 1:30 in the afternoon (New York time). That flashback scene could not have been later than 4:30 because it was still light out (note the light coming through the windows in the background) and sunset in the New York area at that time of year is 4:30 p.m. Its highly unlikely that Mike would have had time to enlist and have been back in time to attend Vito's party. The dead giveaway was Tessio's statement that 30,000 men enlisted that MORNING.