If you're in a chain restaurant, such as Red Robin, and a customer has a birthday the staff will come out and perform for the honored guest. But they won't use the song all of us sing at home. The reason is the restaurant doesn't want to pay copyright royalties.
The company that owns the copyright, Warner/Chappell, has been collecting up to $2 million a year for commercial uses of the song (such as appearing in a movie or publishing a piece made of variations on the tune). They have owned the copyright since 1935. Jennifer Nelson created a documentary about the tune and Warner/Chappell required a payment of $1500 because the song appeared in the documentary. The movie no doubt included a few details about the origin of the movie and Nelson suspected the copyright expired. So she sued.
Warner/Chappell was supposed to supply details of their claim a year ago as part of the case's discovery process. But they just released a few more documents, saying they "mistakenly" forgot to release them last year. The new documents are from 1927 and before and they show the famous tune as not being under copyright, but in the public domain. What Warner/Chappell had copyrighted was a particular setting of the tune, not the tune or lyrics. So sing away without worrying about breaking the law (not that most of us did).
A lot of people are watching this case. Warner/Chappell may have to refund a lot of improperly collected royalties.