The summer solstice occurs on June 20 or 21, when the sun is at its greatest angular distance north of the equator. The summer solstice marks the beginning of summer.
The winter solstice occurs on Decmber 21 or 22, when the sun is at its greatest angular distance south of the equator. The winter solstice marks the beginning of winter.
During the summer in the northern hemsiphere, the north pole is tilted toward the sun. During the winter in the northern hemisphere, the north pole is tilted away from the sun.
The seasons are caused by the tilting of the earth's axis of rotation as the earth orbits around the sun. During the spring equinox (March 20 or 21) and the fall equinox (September 22 or 23), the sun shines directly over the equator, and all parts of the world have equal day and night.
During the summer, days are longer than nights. During the winter, nights are longer than days.
The seasons are reversed in the southern hemisphere.