This lyric is from the song "Last Christmas" by Wham. Note that the Christmas number one vary by time and also location. For example, The Christmas number one single refers to the song that holds the top position on the music charts during the Christmas week, usually in the United Kingdom. The specific song that achieves this position can vary from year to year and from country to country. Different charts exist, and what's considered the "number one" can be different based on regional or national music charts.
In the UK, there's often a race for the Christmas number one spot, and it has become a tradition for artists and fans to campaign for a particular song to top the charts during the festive season. The popularity of a song, special releases, and various promotional efforts can influence which song claims the Christmas number one spot in a given year. The Christmas number one single in the UK for Wham! was "Do they know It's Christmas?" in 1984.