The longest day of the year, also known as the summer solstice, happens when Earth's axial tilt is such that one hemisphere is tilted closest to the sun. In the Northern Hemisphere, this occurs around June 21st, marking the official start of summer. On this day, locations in the Northern Hemisphere experience the most daylight hours and the sun reaches its highest point in the sky.
Conversely, in the Southern Hemisphere, around December 21st, the summer solstice occurs. During this time, the Southern Hemisphere is tilted towards the sun, resulting in the longest day and the official beginning of summer in that part of the world.
The equinoxes, around March 21st and September 23rd, are the points in the year when day and night are approximately equal in duration, marking the start of spring and autumn, respectively.