Godfather Question

I am a huge fan of the the movie " The Godfather" and have seen it countless times. I have also read the book several times. During the last time I watched the movie I had a serious revelation about the content and plot in the opening wedding scene. Specifically: Why was Michael not at the Wedding Ceremony? Michael shows up after the reception is well under way and it is clear from the reaction of Tom and Fredo that they are seeing him for the first time in a while. Also, he is not in a formal military kit but regular uniform. Hence, it is obvious that Michael was not at his Sister's marriage ceremony but only showed up for the reception

This absence seems extremely odd given the cohesive nature of "the family" which is a core theme of the movie and just really puzzles and perturbs me. No one has offered a compelling explanation. Did Coppola have a reason for this cinematic construct or is it just a mistake?