Short answers, pulled from the story.
Western Christianity inherited its practice of starting a new day at sunset from Jewish liturgical tradition, based on the Genesis verse "And there was evening, and there was morning - the first day." Because tradition holds that Jesus was born at night, based on Luke 2:6-8, Midnight Mass became the defining ritual marking the beginning of Christmas.
On the 24th of December 1914, German troops near Ypres, Belgium, began decorating their trenches and singing Stille Nacht. British troops across No Man's Land sang carols in response, and the two sides eventually crossed No Man's Land to exchange gifts and mourn the dead together. Soldiers from both sides read a passage from Psalm 23 at a joint funeral. The truce took place despite opposition from higher military command and occurred on both Christmas Eve 1914 and 1915.
On the 24th of December 1968, astronauts Bill Anders, Jim Lovell, and Frank Borman read aloud from the Book of Genesis while orbiting the Moon, in what was the most watched television broadcast to that date. Bill Anders also took the photograph Earthrise (NASA image AS8-14-2383HR) on that night. In 1969, the United States Postal Service issued a stamp, Scott number 1371, commemorating the flight and featuring that photograph.
Nochebuena translates as "the Good Night" in Spanish, reflecting the tradition that Jesus was born at night. The same idea appears in German, where Christmas Eve is called Heiligabend or Heilige Nacht, meaning Holy Night, and in the carol Silent Night, Holy Night, which is called Stille Nacht in German.
During the Reformation in 16th- and 17th-century Europe, many Protestants moved the date of gift-giving from the 6th of December, the eve of Saint Nicholas' feast day, to Christmas Eve. They also changed the gift-bringer from Saint Nicholas to the Christ Child, known in German as Christkindl.
Wigilia is the Christmas Eve vigil supper in Poland and is the most eventful day of the Christmastide there. According to the Etymological Dictionary of the Polish Language by Aleksander Bruckner, the traditional number of dishes was tied to social class: peasants served 5 or 7, the gentry 9, and the aristocracy 11. Today 12 meatless dishes are common, representing the Twelve Apostles. The custom of sharing a wafer as a gesture of reconciliation arrived in Poland in the 18th century.