That's his official birthday, but his Un-Official birthday is March 13...Atleast according to the Disney short "Donald's Happy Birthday."
That's just the fictional birthday of the character Donald Duck, that the actor Donald Duck portrayed in that film. June 9th is the real Donald Duck's birthday.
It's sort of like the Seinfeld show. The birthday of the character of Jerry Seinfeld in the show was fictional. The actor Jerry Seinfeld that played the character Jerry Seinfeld...his birthday was different.
(Of course most of all of the above is fiction anyway!)
"Who’s got the sweetest disposition?
One guess, that’s who?
Who’d never, ever start an argument?
Who never shows a bit of temperament?
Who's never wrong but always right?
Who'd never dream of starting a fight?
Who get stuck with all the bad luck?
No one (grumble) but Donald Duck (quackYEAHquack)!"