What is the name of the longest day of the year in the Northern Hemisphere?

The longest day of the year in the Northern Hemisphere is the summer solstice.

Astronomically, the summer solstice occurs when Earth's Northern Hemisphere is tilted most directly toward the Sun. The Sun reaches its highest apparent midday position north of the tropics, producing the year's longest period of daylight and shortest night for that hemisphere.

The Northern Hemisphere's summer solstice is also called the June solstice and normally occurs on June 20 or June 21 in the Gregorian calendar. Its date can vary slightly because calendar years and Earth's orbital cycle do not match perfectly. At the same time, the Southern Hemisphere experiences its winter solstice, with its shortest day.

“Midsummer” is a common cultural name for the season or festival surrounding the event, but it is not as precise an astronomical term. The equinox is different: it is when day and night are approximately equal. Another common misconception is that the solstice must produce the earliest sunrise and latest sunset; those usually occur on different dates because of Earth's axial tilt and orbital timing.