"Happy Birthday, to You" also known more simply as "Happy Birthday", is a song that is traditionally sung to celebrate the anniversary of a person's birth, it first appeared in the episode "Puffless". It was written by Mildred Hill and Patty Smith Hill.
Lyrics from Happy Birthday from Sesame Street Edit
The Simpsons: Happy birthday to you,
Happy birthday to you,
Happy birthday from Sesame Street,
Happy birthday to you!