On the 21st of June the world will mark the solstice, the longest day of the year in the northern hemisphere, shortest day in the southern hemisphere. On December 21 the situation is exactly reversed. And in between each of those dates - on March 20 and September 22 (of 2009), the sun's north-south ecliptic path crosses the equator and day and night are of equal length. We call those equinox. Each of these days marks the beginning of a season by the modern western calendar. The days vary year to year between the 20th and 23rd day of the change months.
The word "solstice" comes from the Latin sol for sun, and sistere, to stand still. This term notes that on the days of solstice the sun's apparent annual path between north and south comes to a stop before reversing direction. Our planet's north-south axis of rotation is tilted about 23.44º from the perpendicular of the orbital plane of revolution around the sun. It maintains this tilt through its annual journey around the sun, which causes the hemispheres to vary their relative inclination toward the sun.
Because the seasons are marked by the solstices and equinoxes, and marking seasons has always been important for agriculture, most cultures and religious traditions honor these days with celebrations, rites or festivals of some kind. For the June solstice, Christian and Pagan cultures observe the feat of St. John, St. John's Eve,l Ivan Kupala Day or Midsummer. The December solstice marks a holiday season cultures honor. Christmas, Yalda, Saturnalia, Hanukkah, Kwanzaa, Yule, Karachun are the most notable. The equinoxes have their spring and autumn observances as well, and in some cultures the midpoints between these - called cross-quarter days - are also celebrated.