From A Charlie Brown Christmas, this is the beautiful little song that the children sing as the story begins. The lyrics were written by Lee Mendelson, and the music by Vince Guaraldi.
If you want to listen to a bit of this to get the tune in your head, you can get a sample on the Amazon listing for the soundtrack.
Christmas Time is Here
Christmas time is here
Happiness and cheer
Fun for all, that children call
Their favorite time of year
Snowflakes in the air
Olden times and ancient rhymes
Of love and dreams to share
Go here for the rest of this song.