Visual: Winter Solstice, Hanukkah, Christmas, Kwanzaa, and Gurnenthar's Ascendance
Many religions have a celebration around the time of the Winter Solstice (the shortest day of the year in the Northern Hemisphere, which falls between 20-23 of December). The few mentioned on Giles' banner in the Magic Box were:
Christmas: A Christian festival celebrating the birth of Jesus Christ, on December 25th.
Hanukkah: An 8-day Jewish celebration commemorating the victory of the Maccabees over Antiochus, the king of Syria, in the 2nd century BC. It begins on the 25th day of Kislev. This fell on December 22nd in 2000.
Kwanzaa: Created in 1966 by African-Americans to promote their needs, in response to Christmas, which they felt did not represent them. It is celebrated between December 26th and January 1st.
Gurnenthar's Ascendance is apparently made up.