Huh? you exclaim, before asking: How is it that a character of fiction, cooked up in the late 19th century by Arthur Conan Doyle, a Scottish-born doctor turned author, can have a birthday that thousands of people celebrate every year?
Let me refer you to a Web site operated by the Madison, Wisconsin-based Notorious Canary-Trainers, “an officially-recognized scion of the Baker Street Irregulars,” the worldwide Holmes fan organization (founded in 1934 by American author Christopher Morley). Within their site, the Canary-Trainers feature a page headlined, “Clearing Up a Mystery: Why Do We Celebrate Jan. 6 as Sherlock Holmes’ Birthday?” The answer may seem a bit dubious, even to those who insist that Holmes was a real historical figure, merely introduced to the world by Conan Doyle, not created by him. But in celebration of the Great Detective’s birthday, let me quote from that page here:
The actual date of [Holmes’] birth is murky at best because the written evidence does not exist or, if it exists, has yet to be found.By the way, Morley gave the full date of Holmes’ birth as January 6, 1854. Which would have made him 33 years old (five years older than Conan Doyle) when his first adventure, A Study in Scarlet, saw print in Beeton’s Christmas Annual in November 1887. Dr. John H. Watson was then either 34 or 35, depending on your source.
For the non-believers, it’s lacking because Sherlock Holmes is a fictional character who was born only in the mind of Arthur Conan Doyle. For the believers, however, the lack of evidence is a testament to the poor record-keeping of such matters in the mid-19th century in rural England.
As one Sherlockian scholar once remarked, as the best evidence, “Mr. Holmes’ obituary has not appeared in the London Times, so he can not be dead.”
For those in the believers’ group, here’s perhaps the key argument in favor of January 6 (as summarized by one of our members, Tom Drucker):
1) The Valley of Fear  starts on the 7th of January.
2) Holmes seems to be a little cranky and out of sorts at the beginning of the story.
3) Why would he be cranky and out of sorts? He must have a hangover.
4) Why would he have a hangover? He must have been celebrating the night before.
5) What could he have been celebrating? Surely it was his birthday.
That argument also is the one made by William S. Baring-Gould, who produced the first annotated Sherlock Holmes collection, plus other writings on Sherlock Holmes.
He also makes a second argument, as follows:
Holmes quotes Shakespeare often, but the only play he quotes twice is Twelfth Night. Twelfth Night is January 6. It must be his favorite because it coincides with his birthday.