I could be wrong, but most people here in the States don't refer to "summer" and "fall" via dates; it's more tied to school and Labor Day. Labor Day (first Monday in September, this year: September 7) usually signifies the 'end of summer' and most schools go back just before or immediately after (my town public school, Thursday, September 3, my daughter's private school, Wednesday September 9) Labor Day.
So I can imagine John thinking there is another week or so of "summer".