The summer solstice is the longest day of the year in the Northern Hemisphere, occurring when the sun's apparent path is farthest north of the equator. This astronomical event takes place either on June 20th or 21st. The summer solstice is a result of the Earth's axial tilt and its orbit around the sun, causing the Northern Hemisphere to be tilted most directly towards the sun. This leads to the sun reaching its highest point in the sky, resulting in maximum daylight hours. 
Key aspects of the summer solstice:
Longest day:
The summer solstice marks the day with the most daylight hours in the Northern Hemisphere. 
Earth's tilt:
The Earth's axial tilt (approximately 23.5 degrees) is the primary reason for the solstice and the changing seasons. 
Tropic of Cancer:
In the Northern Hemisphere, the sun's rays are directly overhead at the Tropic of Cancer during the summer solstice. 
Summer's beginning:
The summer solstice signifies the start of the astronomical summer in the Northern Hemisphere. 
Cultural significance:
Many cultures celebrate the summer solstice with festivals, rituals, and ceremonies. 
In more detail:
The Earth's tilt on its axis is not always pointing directly towards the sun; it shifts throughout the year. During the summer solstice in the Northern Hemisphere, the North Pole is tilted most directly towards the sun. This means that the sun's rays are more concentrated on the Northern Hemisphere, leading to warmer temperatures and longer days. The opposite is true for the Southern Hemisphere, where the winter solstice occurs with the least daylight hours. 

The term "solstice" itself comes from the Latin words "sol" (sun) and "sistere" (to stand still), describing how the sun's apparent position in the sky seems to pause at its highest point before reversing direction.