Short answers, pulled from the story.
A solstice is the moment when the Sun reaches its farthest point north or south of the celestial equator before reversing direction. Two solstices occur each year, around the 20th-the 22nd of June and the 20th-the 23rd of December.
Solstice comes from the Latin solstitium, combining sol (sun) and -stitium (stoppage), because at the solstice the Sun's vertical motion in the sky appears to halt before reversing. The word entered English during the Middle English period and was already in use during the late Roman Republic of the 1st century BC.
The seasons result from Earth's axis being tilted about 23.44 degrees from perpendicular to its orbital plane. The solstices are the two moments each year when that tilt has maximum effect: at the June solstice the Sun shines directly over the Tropic of Cancer at 23.44 degrees north, and at the December solstice it shines over the Tropic of Capricorn at 23.44 degrees south.
Greek astronomers used an interpolation method, recording the Sun's noon declination on two symmetrically placed days before and after the solstice and finding the midpoint between matching readings. An interval of forty-five days was identified as the best spacing for achieving up to a quarter-day precision. In 1276, Chinese astronomer Guo Shoujing built a 12.6-meter gnomon at the Gaocheng Astronomical Observatory, producing a measurement of the year's length within one minute of the value later used in the Gregorian Calendar.
The December solstice is associated with Christmas, Yalda, Saturnalia, Karachun, Hanukkah, Kwanzaa, Yule, and the East Asian Dongzhi Festival. The June solstice is marked by the Christian feast of St. John from the 23rd to the 24th of June, the Wiccan Litha, the Aymara New Year on the 21st of June at Tiwanaku, and the Mapuche We Tripantu on the 24th of June. Antarctic research stations including McMurdo Station and Halley VI celebrate the 21st of June as Midwinter Day.
Due to the precession of Earth's rotational axis, the solstice points drift slowly through the constellations over thousands of years. The northern solstice moved into Taurus in December 1989 and is expected to enter Aries in the year 4609. The southern solstice is expected to pass from Sagittarius into Ophiuchus in the year 2269.