Lyrics
The crew singing the song to Nibbler
"What day is today?
It's <Clint> 's birthday.
What a day for a birthday!
Let's all have some cake."
This song, a parody of the traditional birthday song, first appeared in "I Second That Emotion" and was sung to Nibbler in celebration of his 5th birthday.
In the episode "The Late Philip J. Fry", it was also sung by Elzar to Leela on her birthday.