In the United States, summer officially begins in June. The month contains the Northern Hemisphere's June solstice, the astronomical event commonly used to mark the start of summer.
The exact solstice date varies slightly, occurring between June 20 and June 22, most often on June 21. At the solstice, the Northern Hemisphere is tilted most toward the Sun, producing its longest daylight period and shortest night of the year. Afterward, daylight gradually begins to decrease even though summer has just started.
There are two common ways to define the season. Astronomical summer begins at the solstice, while meteorological summer begins on June 1 and runs through August 31 for easier climate comparisons. Both systems place the start in June, so the month remains the single canonical answer.
A common mix-up comes from the Southern Hemisphere, where June marks the beginning of winter rather than summer.