Short answers, pulled from the story.
Two solstices occur annually around the 20th, the 21st, and the 22nd of June and between the 20th and the 23rd of December. The dates vary depending on the time zone but always fall within these ranges in a four-year cycle.
The word solstice derives from the Latin solstitium containing two morphemes: sol meaning sun and -stitium meaning stoppage. Pliny used the term in his Natural History with a similar meaning to its modern usage before it entered English during the Middle English period.
Earth's axis makes an angle of about 23.44 degrees relative to its orbital plane known as the obliquity of the ecliptic. At maximum elevation the relative yearly motion of the Sun perpendicular to the horizon stops and reverses direction while the subsolar point reaches latitude 23.44 degrees north or south.
Christmas is the most widespread contemporary holiday celebrated around the winter solstice alongside Yule Saturnalia Karachun Hanukkah Kwanzaa and Yalda. In East Asian cultures the Dongzhi Festival is also celebrated on the winter solstice.
During the June solstice places on the Arctic Circle will see the Sun just on the horizon during midnight while all places north of it experience 24 hours of sunlight. This phenomenon is called the midnight sun midsummer-night sun or polar day.