The sun is highest in the sky and daytime is longest during the summer solstice.

This occurs around June 20th or 21st in the Northern Hemisphere and around December 21st or 22nd in the Southern Hemisphere.

Here's why:

* Earth's Tilt: The Earth is tilted on its axis. This tilt is the reason we have seasons. During the summer solstice, the hemisphere tilted towards the sun receives the most direct sunlight, resulting in the longest day and the sun's highest point in the sky.

* Solar Declination: The sun's apparent position in the sky changes throughout the year. During the summer solstice, the sun reaches its northernmost or southernmost declination (depending on the hemisphere), leading to the longest day.

Let me know if you have more questions!