The accumulation of Christmas melodies that we now have available to us is amazing. From the simple Gregorian chant-like melodies of the earliest extant music to hymns meant to be sung by a church congregation to the modern tunes in many styles that are still being contributed to the repertoire today, Christmas songs form a unique collection that spans all moods and genres.
I've always liked this seasonal music, so much so that I made my own collection of melodies (something like 180 of them so far), taken from a variety of sources, so that I can entertain myself by playing these favorites on flute or clarinet. One characteristic of Christmas songs is that they are almost all meant to be sung. This might sound like a "DUH - how obvious" statement, but there are many forms of music that are not a simple melody line with accompaniment. I doubt that many people go around humming tunes from the latest Rap or Punk Rock album. You can't play a Beethoven symphony on a single wind instrument and you can't sing the Anvil Chorus without the, well, chorus. However, from the beginning, Christmas songs were created so that an untrained voice could sing them. Most are easier to sing than our national anthem.
Because I'm an instrumentalist, I fall into the usual description of such in that I don't know how to dance (I'm usually playing in the band) and I don't know the words to songs. That's a bit of a simplification - I know SOME of the words to songs, but rarely all of them. In fact, for a good number of Christmas songs, all I know is the first line, which happens to conveniently double as the title of the song. Sometimes I've found tunes I like and/or remember from my childhood in books of non-English origin. Then, all I know is the title in another language, which often has the words "Jesus," "Christmas," or "Bethlehem" in some form or other.
The origins of some songs I know are totally foggy. I'm not sure which country they are from, whether or not they were composed by somebody that wanted to be remembered for doing so, or even what their correct title is. I don't really care, since I just find the melodies to be fun to play and hear.
Part of the joy of Christmas songs is the way different styles can be juxtaposed and still make a cohesive collection. I'm partial to songs in minor or modal keys (i.e. "Coventry Carol" or "Sing We Now of Christmas"), maybe because they are not very numerous. After all, the point of these songs is to make people feel upbeat, and minor keys are not usually the obvious choice for that. I like the ones that are in a "soft-shoe" style ("It's Beginning to Look a Lot Like Christmas" and "Christmas Eve in My Home Town"). And then I like the ones that are cheery and upbeat ("Sleigh Ride" and "Jingle, Jingle, Jingle"). During World War II, some very wonderful songs played up the sentimentality of the holiday ("Have Yourself a Merry Little Christmas" and "I'll Be Home For Christmas"). Songs range from deeply devotional ("Silent Night" and "Oh Little Town of Bethlehem") to silly and frivolous ("Suzy Snowflake" and "Frosty the Snowman").
For a song to be a Christmas song, it need only refer to anything to do with winter. While a lot of songs do relate to Jesus, Mary, and Bethlehem, a good number also concentrate on Santa Claus, trees, gifts, mistletoe, snow, sleighs and reindeer. There are also a smattering of those celebrating virtues, like "Let There Be Peace On Earth."
Perhaps one of the best things about my collection of Christmas tunes is that they are by many different composers. Some of the writers are so famous that just about everyone has heard of them (who doesn't know that Irving Berlin wrote "White Christmas?"). A good many songs are by the well-known Anonymous. Some composers are just a name on the page and I've never heard of them in any other context. While the basic melodies of most Christmas songs are quite simple (remember, they are intended for ANYBODY to be able to sing), one really takes the cake. Only a masterful composer like George Frideric Handel could create a popular and enduring Christmas carol that starts with nothing more than the eight notes of a descending diatonic scale. The song? "Joy to the World."