Answer: Autumn
It must be fall, because the friends are hunting, and because football is being shown on TV in the bar just before the wedding.
The football game is between the Philadelphia Eagles and the Pittsburgh Steelers. During the 1968-75 period covered by the film, all games between those two teams were played on Sundays. At the reception, Michael says they leave for Vietnam on "Monday" - and the friends have one last hunting trip the day after the wedding. So the timing of the wedding, hunt, and Vietnam departure isn't exactly correct.
Details, details.