I've always liked Leroy Anderson's Sleigh Ride as a secular Christmas song :) It's not technically a Christmas song since the words never mention Christmas but it's often played now so it seems like a way to ease into the season.
Anderson had the original idea for the piece during a heatwave in July 1946; he finished the work in February 1948. Lyrics, about a person who would like to ride in a sleigh on a winter's day with another person, were written by Mitchell Parish in 1950.
The orchestral version was first recorded in 1949 by Arthur Fiedler and The Boston Pops Orchestra. The song was a hit record and has become the equivalent of a signature song for the orchestra.
A fun arrangement has been made for piano duet.