Go foo-bar, it's your birth-day.
A phrase used to congratulate or cheer on someone. While it certainly makes appearances during birthdays, it doesn't actually have anything to do with whether or not it's your birthday; it's just a cool (or at least popular) way of saying "Yay for you!".
Go Foxxy! It's your birthday!
Not for real real, just for play play!
-- Foxxy Love, Drawn Together season one, 2004*.
Although it is usually associated with African American and hip-hop culture, it has been used by white folk for decades. Its primary musical usage has been in rap and hip-hop songs, and its distinct cadence does hint at a musical origin. Its exact origins are lost in the mists of time, but it gained its first big boost into the vocabulary of American popular culture when Uncle Luke (formally of the 2 Live Crew) came out with It's Your Birthday:
Go Shela, it's your birthday
Go Shela, it's your birthday
Go Ferdy, it's your birthday
Go Ferdy, it's your birthday
It's your birthday, get busy
It's your birthday, get busy
It's your birthday, get busy
It's - your - birthday!
-- Uncle Luke, Freak for Life, 1994
Calling it a hit would be stretching it, but it was the perfect song for DJs to play when it was someone's birthday, and despite being a bit repetitive, it was a pretty catchy song. It got enough radio play that most children of the 90s would recognize it. (Although I find that many people don't have any recollection of it until they actually hear it played again). This is when the phrase really became 'cool', although both "Go ___, it's your birthday" and various rejoinders such as "No it's not, do it anyway!" (which did not appear in the song) had already been in use for years. Of course, when most people think of the phrase 'it's your birthday', they don't think of Uncle Luke; they think of 50 Cent.
Go - Go - Go - Go - Go - Go
Go, shorty it's your birthday.
We gonna party like it's your birthday.
We gonna sip Bacardi like it's your birthday.
And you know we don't give a f*?@ cus it's your birthday.
-- In Da Club, 50 Cent, Get Rich or Die Tryin, 2002
This was 50 Cent's first number one hit single in the USA, and has since been pirated, remixed, and parodied all over the world. The phrase 'go, ___, it's your birthday' had been heard in the UK prior to the release of In Da Club, but before this it had not really been popular anywhere outside of the US. 50 Cent also cemented "Go shorty" to the phrase, apparently a new innovation. In 2006 Luther Campbell (AKA "Uncle Luke") filed a lawsuit against 50 Cent, claiming that the phrase "go __, it's your birthday" was lifted from his song, and that he was therefor owed royalties. The judge ruled that the phrase was not eligible for copyright protection, being a "common, unoriginal, and non-copyrightable element of the song". No specific conclusions were drawn as to the origin or history of the phrase.
While dictionaries of idiom and slang fall short, perhaps E2 could have a shot at finding the earliest appearances of this phrase. I have found hints that it might be heard on old episodes of the "Martin" show, or the movie Who's the Man? (1993). I don't have access to these, but someone out there must -- and I'm sure there are other historical references out there that the sharp eyes and ears of my fellow noders can root out. /Msg me with any insights or discoveries!
* The play-play variation did not originate with Drawn Together; it too has been around since the 90s.
References:
http://ask.metafilter.com/25109/Its-Your-Birthday-Have-a-Party
http://www.imdb.com/title/tt0386180/quotes
http://sohh.com/articles/article.php/10117 (lawsuit)
http://board.deathvalleydriver.com/index.php?showtopic=9273&pid=170727&mode=threaded&start= (lawsuit)
http://en.wikipedia.org/wiki/In_da_Club
http://www.amazon.com
Urbandictionary.com