Correspondent Scott Carr says there’s a good reason why some of the most popular songs of the season are sold. Of the most “covered” Christmas songs on Spotify (songs that are rerecorded by an artist other than the original), only two of the top 25 were written after 1945: Silver Bells and Elvis’ “Blue Christmas.” In fact, most of the best-liked Christmas covers were originally popular before the 20th century. The most popular song, Silent Night, was written in 1818. The oldest song, We Wish You a Merry Christmas, was written in 1500 A.D. Part of the reason so many old songs are rerecorded so often is because when the songs’ copyrights expire, the become “public domain,” meaning artists can record them without paying royalties. Currently in the United States, all music published before 1923 is part of the public domain.