I grew up in a family that could sing in harmony, if we so desired. But, with the 'Happy Birthday' song especially, it was usually most fun to see how out of tune and tempo and loud we could be. It just made it seem more festive to sing with gusto!
Now, in my little family, we have a nightly tradition of kneeling in family prayer, and we sing a song to let the children (or parents) know that it is time to gather in a circle. My husband is blessed with perfect pitch. I'm blessed with being able to sing out of tune and enjoy the 'interesting' harmonies. (Or should I say dissonance?) He used to join me in whatever key I happen to be singing in. I like to sing alto and that song is in the soprano range.
When he is the one to start the song, (hardly ever!), then he sings it in the right key.
Lately, he tries to sing it in the right key no matter what key I sing in. It's actually become quite fun for the kids and I to watch his pained expression as we are all singing whatever notes we want, while he tries hard to stay in the 'right' key.
He compared it to living the gospel, trying to stay on the strait and narrow, while the world goes wide and crooked.