The longest day of the year is called the summer solstice. It occurs when one of Earth's poles has its maximum tilt toward the Sun, resulting in the longest period of daylight and the shortest night of the year in that hemisphere. For the Northern Hemisphere, the longest day usually falls between June 20 and June 22, with 2025's summer solstice occurring on June 21. On this day, the North Pole is tilted toward the Sun, and the Sun reaches its highest position in the sky. The Northern Hemisphere experiences the most daylight hours, with locations farther from the equator having significantly longer days. For example, in the UK, the sun can be up for over 16 hours during the summer solstice. Conversely, in the Southern Hemisphere, the same day marks the shortest day of the year as it experiences its winter solstice. The summer solstice marks the start of astronomical summer in the Northern Hemisphere. It is the peak of the Sun's annual march higher in the sky before it starts moving lower and days become shorter again. In short: the longest day of the year is the summer solstice, occurring around June 21 in the Northern Hemisphere, featuring the longest sunlight hours of the year.