Longest day of the year is actually at the beginning of summer, and this year it was June 20th. Days have been getting shorter since then.
Sidebar: That's astronomical summer, and absolutely correct.
Meteorologists define the seasons differently; meteorological summer is June-July-August. Since the subject is the heat, this seems a more appropriate application. As such, it's still not the "middle" of summer, but it is well into it.
More casually, in the United States, many refer to summer as Memorial Day weekend through Labor Day.
None of which matter a lot because, however what you call it, playing all day long on the longest day of the year, when it's generally pretty hot in most places, requires a lot of hydration and some nutrition. Though the former is the real important part---lack of hydration can be life threatening. For most of us, lack of food may be miserable but won't lead to a 911 call; a wide range of snacks will work, to varying degrees. Myself, I rarely carry snacks but when I must, I prefer things that are grazable, a little bit at a time, like trail mix, nuts, or dried fruit.