The summer solstice (in Latin, sol, “sun,” and stice, “come to a stop”) is the day of the year with the most daylight. The first day of the astronomical Northern Hemisphere summer is the day of the year when the sun is farthest north (on June 20 or 21). In 2012, this occurs on June 20 at 6:09 pm CDT.
As Earth orbits the sun, its axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of Earth’s axis to the sun is always changing.
As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude. This links the amount of solar energy reaching a location to the time of year and causes some months of the year to always be warmer than others — in other words, the seasons.
On the Northern Hemisphere’s summer solstice, the northern spin axis is tilted toward the sun, and latitudes north of the Arctic Circle (66.5 degrees N) have 24 hours of light. A common misconception is that Earth is closest to the sun in our summer. Actually, Earth is closest to the sun in December, which is winter in the Northern Hemisphere.
The summer solstice is often referred to as the first day of summer but there are other definitions of summer. For example, culturally we can consider summer to begin during the Memorial Day holiday weekend or after school gets out.