Since you knew it was American centric, and you knew when autumn or "fall" or summer are in the Northern Hemisphere, where all† of the United States of America‡

Since you knew it was American centric, and you knew when autumn or "fall" or summer are in the Northern Hemisphere, where all† of the United States of America‡ is located, You did, in fact, know when was referred to. It's even possible to realize that "over the summer" can include Christmas in Sydney or Buenos Aires! Mostly people realize we live on a sphere, and what the consequences of that are. † With apologies to American Samoa. ‡ Country name spelled in full to avoid the predictable bellyaching about the existence of a continent of the same name, or two, depending on where you hail from. Another thing you are not in fact confused about.