I checked an old CC posting asking the same thing and here was the response (from an “Admissions Expert”):
Here’s what I recommend: The activity should be tacked onto the end of the grade just completed unless you start it toward the end of the summer, then you can tack it onto the NEXT grade.
There really is no “right” or “wrong” answer to this.
So this suggests that you can credit it to either year and be correct - just be consistent.