Directors shoot films as their resources and timetables allow or can be arranged - actors, places, equipment, weather, etc. However, I've never known of a Communion taking place at any time other than in the Spring. On the other hand, Ola et al arrive at the Communion wearing overcoats.
But it's also the director who decides when his film takes place. The screenplay says it's Autumn.
And believe me, if Michael Corleone wanted the communion to be held in Autumn, it would have been held in Autumn. If he could order the death of his brother and an arch bishop, he could as easily order the death of Father Carmelo, or sent Al Neri after him to give him an ass whipping if he would disagree.
