"Christmas Comes But Once a Year" is the original song from the Fleischer Studios short of the same name. It is sung by Grampy and the Orphans at the Ophanage, and it is repeated at the finale sequence.
Christmas comes but once a year
Now it's here, now it's here
Bringing lots of joy and cheer
Tra la la la la
You and me and he and she
And we are happy now because
Because, because, because, because
There is a Santa Claus!
[Chorus, with "Yes," added at the first line and the last "la" ends in a high note at the last line.]