Christmas, Dec 25: Christian holiday commemorating the traditional birth-date of Jesus. Also celebrated as a secular gift-giving holiday; other observances include the decoration of trees & houses.
Hanukkah, 26 Kislev-2/3 Tevet--almost always in December: Jewish holiday celebrating the defeat of Seleucid forces who had tried to prevent Israel from practising Judaism, and also celebrating the miracle of the Menorah lights burning for eight days with only enough (olive) oil for one day.
Kwanzaa, (created in 1966) Dec 26-Jan 1: Holiday observance honoring African-American heritage, primarily in the US.

