A few years ago I bought a great piano book compiling classical holiday songs from Baroque to early 20th Century composers. One thing I've always been good at as a musician is reading music, so once I got the book home, I opened it up and started "reading" it from cover to cover, playing through all the songs. It was a lot of fun, especially since I liked more songs than I disliked, but once into the romantic era I became confused. Why did all these songs sound like "Twinkle Twinkle Little Star?" Were the editors stuck and needed filler material so they figured Christmas, there was that star, it probably twinkled, so let's throw in a bunch of "Twinkle Twinkle Little Star" variations?
Turns out that this is a popular tune with many lyrics set to it, including the alphabet song and "Baa Baa Black Sheep." The tune is originally by M. Bouin published in Les Amusements d'une Heure et Demy as a recreational piece without lyrics. In France, the song is called "Ah, vous dirai-je, Maman" and is about a child telling the mother how the father would like the child to use grown-up reason but the child finds sweets worth more than reason. In Germany, it became a song about Santa bringing presents entitled "Morgen kommt der Weinachtsmann." The trivia gets crazier, as the lyricist is the same one who penned the German national anthem, Hoffmann von Fallersleben. Ernö Rossa wrote another song about Santa using the same melody. This one is called "Hull A Pelyhes Fehér Hó." Another Christmas song uses this tune in Spain, "Campanita del Lugar." The song is about ringing the "little town bell" because Jesus is born. Unrelated, the word for Christmas Eve in Spanish is "Nochebuena." "Noche" is the word for night and "buena" is the word for good. So literal, Christmas Eve is a good night.
There are other countries who use the melody of "Ah, vous dirai-je, Maman" for non-Christmas songs as well, but believe me that if you ever hear me playing a song with the melody of "Twinkle Twinkle Little Star," I am obviously playing, "Campanita del Lugar" and definitely not the Hungarian or German Christmas songs.