Short answers, pulled from the story.
The solstice is the precise moment when the daily path of the Sun across the sky reaches its northernmost or southernmost limit and appears to pause before reversing direction. This phenomenon derives its name from the Latin solstitium meaning sun and stoppage and was observed by ancient astronomers who saw the seasonal movement of the Sun's daily path reach a limit before turning back.
The Earth's axis of rotation makes an angle of about 23.44 degrees known as the obliquity of the ecliptic which is not perpendicular to its orbital plane. This tilt causes the seasons to occur because for half the year the Northern Hemisphere is inclined toward the Sun while for the other half year the Southern Hemisphere has this distinction.
The solstices always occur between June 20 and 22 and between December 20 and 23 in a four-year-long cycle with the 21st and 22nd being the most common dates. The dates of the solstice vary each year and may occur a day earlier or later depending on the time zone because the earth's orbit takes slightly longer than a calendar year of 365 days.
The constellations the sun appears in at solstices are currently Taurus in June and Sagittarius in December despite the astrological signs that the sun is entering being rooted in Roman Classical period dates. The northern solstice passed from Leo into Cancer in year -1458 passed into Gemini in year -10 and passed into Taurus in December 1989.
During the southern or winter solstice Christmas is the most widespread contemporary holiday while Yalda Saturnalia Karachun Hanukkah Kwanzaa and Yule are also celebrated around this time. For the northern or summer solstice Christian cultures celebrate the feast of St. John from June 23 to 24 while Modern Pagans observe Midsummer known as Litha among Wiccans.