Halloween may not seem like a Christian holiday, but that is what it originally was.  The word "Halloween" comes from the Old English "Holy" or "Hallowed" or "Hallow." The ending to the word 'een comes from "Evening" or "Een."  Combined you get holy evening or Hallow-een.  The evening was holy because the next day, November 1st is "All Saints' Day" a holiday to thank God for Mary, the martyrs, the Saints and all Christian dead.   The traditions of costumes and trick-or-treat come from traditions in Europe.