25 December 1642. 
Other than the ones that are federal holidays (and while I get the day off I don’t do anything to celebrate even those) I don’t think I could even tell you the birthdate of any celebrities.
My birthday has become a rather significant national date (9/11) so I’m sure at some point I’ve heard of other celebrities born on that date but I don’t remember any.
As others have said, I don’t celebrate my own so figure it’s fair to ignore everybody else’s as well.
As to why do this, the answer seems to be for fun. I don’t celebrate any celebrity’s birthday, but maybe I should.
Hmmmm…I am a psychologist, so maybe I should smoke a cigar on September 23.
Canned haggis is a good substitute. There’s no substitute for a good scotch.
Likewise, but not for any religious reasons: more tradition.
I have always thought about those with 9/11 birthdays. What was once a happy day for them is now a national day of mourning. It must be an odd feeling.
I celebrate a few composers’ birthdays by playing their music all day.
Mickey Mouse on November 18th, the date “Steamboat Willie” premiered in 1928.
Tokyo Disneyland usually puts on a little tribute or birthday greeting for Mickey, so I spend the day at the park and eat some Mickey-shaped meat buns and waffles. Yummy.
“Celebrate” is not the right word, but Gary Busey’s character in the film DC Cab (1983) always took off for Elvis’ birthday on January 8. Ever since then, his birthday has been cemented in my mind.
At least for me, it’s not as I honestly was indifferent to my birthday (a fair portion of my childhood was spend with the Jehovah’s Witnesses and even after I left the habit of caring never took).
And now I have a pretend reason why I don’t do anything.
I had a friend in middle school/high school that I still keep in touch with who would get very upset whenever people would comment on it and imply that she shouldn’t celebrate. I always forget her birthday until the last minute like I do with everyone else. I guess it’s a good thing though to not associate “9/11” with her birthday (people talking about the upcoming anniversary never reminds me).
To answer the OP, I have never celebrated a famous person’s birthday nor has it ever crossed my mind, not even for my most favorite artist.
I am also curious as to who this James Otto guy is.
^^^ Go here to check out the song that James is most known for right now. 
I forgot to mention earlier that I watch my favorite films of my favorite actresses when their birthday is here: Mary-Kate & Ashley.
God bless you and them and James always!!!
Holly (a day one fan of the girls)
Stephen Sondheim and Andrew Lloyd Webber were both born on March 22nd (1930 and 1948, respectively). You betcha damn butt this musical theatre nut celebrates it.
Jerry Mathers. It just happens to be my birthday too.
Ernest Borgnine, for the same reason.
Bob Marley, Jimmy Buffett and Tom Hanks.
I celebrate Bob’s, because, hell, why not. Jimmy’s is December 25 and I often find myself celebrating that day, and Tom and I share a birthday.