The Summer Solstice is the longest day of the year, and occurs on June 21st in the Northern Hemisphere. After the Summer Solstice the days begin to get imperceptibly shorter each day, until the Fall Equinox, when day and night are of equal length. The Winter Solstice is the shortest day of the year, and the Spring Equinox finds the days and nights equal once again. Then the days begin to get longer until another Summer Solstice occurs. These changes in the orientation of the earth and its orbit are what create seasons.
The Summer Solstice occurs on June 20-21 each year. It is considered the official first day of summer in the Northern Hemisphere and of Winter in the Southern Hemisphere. As the earth spins on its axis, it tilts either toward or away from the sun. When it tilts towards the sun, between June and September, it creates summer in the northern hemisphere; from December to March, it tilts away from the sun, creating winter in the Northern Hemisphere.
Many cultures celebrate the solstices and the equinoxes, and many holidays have emerged to commemorate them. Christmas's pagan origins arose out of the Winter Solstice celebration. Other holidays such as Yalda, Saturnalia, Karachun, Hanukkah, Kwanzaa and Yule are also celebrated around this time.