Where does the Happy Birthday song come from and why does practically the entire Western world know the words?
The words were written as a classroom greeting in 1893 by two Louisville teachers, Mildred J. Hill, an authority on Negro spirituals, and Dr. Patty Smith Hill, professor emeritus of education at Columbia University.
The melody was composed by Mildred J. Hill, a schoolteacher born in Louisville, KY, on June 27, 1859. The song was first published in 1893, with the lyrics written by her sister, Patty Smith Hill, as “Good Morning To All.”
source of information: http://www.ibiblio.org/team/fun/birthday/
Why does everybody know it? My guess would be that the words and music are short, repetitive, and easily learned. We all have birthdays, and we all heard it when we were very young.
More info as the Master speaks:
Cecil talks about its origins in this column. He’s also written about it in the past.
As for its universality, it’s just like Christmas carols. It’s a song that’s been passed down over and over and over again. Besides, do you know of any other birthday songs? I didn’t think so. 
Robin
Dang! Must learn to type faster than the Mercotan…
[clears fields sullenly]
Preview:
Hehe, so must Robyn evidently…
No joke, but when you have good research skills like DDG and I do, getting beaten doesn’t matter
Robin
Christmas carols have been around for quite a long time I thought (1600s). U.S. was not the cultural powerhouse in the 1890s it is now. I’m just having a hard time figuring out how the song has become so popular (it is THE birthday song in Argentina and Italy and I’d bet the rest of South America and possibly Europe) around the world in so short a time. Was there a huge radio promotion of the song and even if there were that wouldn’t have happened until many years later when Radio got big? Is the tune that catchy that all of Western Civ would just up and take it as a tradition just like that? Something smells very fishy here…
What I meant was, both kinds of songs are word-of-mouth phenomena. Christmas carols were sung each year and became traditional music for this season. You didn’t need radio, because you had people who traveled and who knew the songs. And, different regions gave us different songs.
“Happy Birthday” went through the same thing. People learned the song, and it spread. IIRC, it was published in a mass-market magazine, which helped a lot.
Robin
Actually, a lot of Christmas carols only date from the 19th Century.
There’s something unsettling Christmas being the topic of your 666th post!
If that’s the most unsettling thing about me… then you haven’t been paying attention!
– Beruang