July 1 is the usual time here. The summer months are best because there usually aren’t as many things underway in the parish (school is out, some parish activities may have stopped for the summer, and the major feast days after Easter are over by this time, etc.), and there is plenty of time for the new pastor to get acquainted with the parish before the busy time of year. I sometimes hear about priests moving in December or January, but not very often- those are usually recently-ordained priests being assigned as an associate pastor or administrator, or a priest retiring and a few more priests moving around to adjust.