People used to have to pay royalties to the copyright holder to be able to sing happy birthday in a commercial setting, but as of September 2015 a judge ruled against the copyright held by Warner/Chappell Music. Since then you’ve been able to sing happy birthday legally without paying for it.
Was Happy Birthday Copyrighted?
“Happy birthday” was copyright protected from 1935 until Sept 2015. During that time you couldn’t sing happy birthday publicly without paying royalties. This is why many businesses sing a variation of the song when they celebrate people’s birthdays.This PBS video from when “Happy Birthday to You” was still copyrighted discusses the background on “Happy Birthday” and it’s copyright.
In 1988, Warner/Chappell Music purchased the company owning the copyright for US $25 million, with the value of “Happy Birthday” estimated at US $5 million. “Happy Birthday to You” had reportedly been generating some $2 million in royalties per year for Warner/Chappell Music. 
- “Happy Birthday to You“. Wikipedia.org. Retrieved Oct 4, 2015.
- “Why the ‘Happy Birthday’ Song’s Copyright Has Finally Been Ruled Invalid” Dailysignal.com. Retrieved Dec 17, 2015.