Why don't temperatures reach their maximum when the sun is at its highest, and days at their longest

The air warms up more quickly than the land and the sea. On June 21st (the longest day), the air loses some of its heat to the land and the sea that are still relatively cool. After June 21st the air temperature is still higher than the land and sea temperatures so they both continue to warm up. This results in the air losing less heat to them. So although the amount of heat the air receives from the sun begins to reduce in July and August, the highest temperatures of the summer are often recorded during these months. In winter a similar effect occurs as the coldest day is often a month or so after the shortest day.