At Doc's birthday the patrons of the Longhorn sing "For He's A Jolly Good Fellow." This song is often used as a substitute in order to avoid paying the license fees for "Happy Birthday To You." In this instance , the use of "For He's A Jolly Good Fellow" is more historically accurate. According to Wikipedia, "Happy Birthday To You" didn't appear in print until 1912. The melody for "For He's A Jolly Good Fellow" goes back (allegedly) to the 18th century in France with the Jolly Good Fellow lyrics added later in the United Kingdom.
Doc has a birthday party in this one, they don't reveal his age, but he was 61.