It is a mystery worthy of a determined sleuth: When exactly is Sherlock Holmes’s birthday? So what if he is a fictional character? So what if Sir Arthur Conan Doyle never explicitly reveals the birthday in his short stories and novellas?

Using logic that is, at best, tendentious, most Sherlock Holmes fans have converged on Jan. 6 as the date of famed British detective’s birth — and thousands of them will converge this weekend to celebrate it. The largest and oldest gathering, held by the Baker Street Irregulars, will take place in New York City, centered at the Algonquin Hotel. Even though Mr. Holmes made his home at 221B Baker Street in London (and there is a London-based fan club), his psychic home may be the literary circles of New York that have embraced the supersleuth with an eccentric charm.

The Baker Street Irregulars gathering, started in 1934 by the literary essayists Christopher Morley and Vincent Starrett, has become incredibly elaborate, featuring a full-dress banquet, an endowed lecture, a theatrical performance, cocktail parties and special breakfasts over several days. The Baker Street Irregulars, which is named for a ragtag group of street urchins that Mr. Holmes occasionally hires in his book, has included among its members Franklin D. Roosevelt, Harry S. Truman and Isaac Asimov.

Until the early 1990s, the organization was open to men only. (In response, women formed the Adventuresses of Sherlock Holmes.) Currently, the group’s membership numbers are near 300 around the world, with people from countries like Japan, Switzerland and Italy.

So why Jan. 6?

Paul Singleton, a Sherlockian scholar and actor in New York, said Christopher Morley made the following argument: Sherlock Holmes quotes Shakespeare often, but the only play he quotes twice is “Twelfth Night.” “He determined that Sherlock Holmes was born on the twelfth night, which is January 6.” (That date would coincide with the night of the 12 drummers drumming from the famed song. The drummers’ labors costs, incidentally, are used to calculate of gifts forms a good consumer price index).

Another argument for Jan. 6 was made by William S. Baring-Gould, who produced the first annotated Sherlock Holmes collection. He and others have argued that “The Valley of Fear,” the final Sherlock Holmes novel, starts on Jan. 7.

Mr. Holmes seems to be a little cranky at the beginning of the story and snaps at Dr. John H. Watson. But why would he be in a bad mood? Because of a hangover. Why would he have a hangover? He must have been celebrating the night before. What could he have been celebrating? Certainly it was his birthday. (This is nebulous at best, from City Room’s perspective.)

Another reason that was offered: Jan. 6 was the birthday of Mr. Morley’s brother Felix.

“It’s nebulous,” Mr. Singleton admits. “There have been alternate dates proposed.”

Some argue Sherlock Holmes’s birthday really should be pegged to Dec. 2, the real-life birthday of Dr. Joseph Bell, the professor Doyle is said to have used an in inspiration for Mr. Holmes.

Others take an even more practical argument for the real-life birth of a fictional character. Mr. Holmes’s first appearance was in a cheap paperback called “Mrs. Beeton’s Christmas Annual,” which was published around Dec. 1, 1887. Others say he was born in Doyle’s imagination, an event that some Holmes scholars presume took place in the fall of that same year, based on the author’s own notes.

The birth year is less up for dispute: 1854. The argument for that? In 1914, Mr. Holmes was described to be a man of 60 when he was instrumental in the capture and arrest of a Prussian spy known as Von Bork. Nonetheless, his centennial was celebrated in 1987, 100 years after he first arrived in print.

Whatever the reason, Jan. 6 more or less stuck as the birth date because of Morley’s insistence as the founder of the Baker Street Irregulars.

“He was the leader,” Mr. Singleton said. “He reinforced it over the years. Eventually everyone just accepted it. God knows why we have to come to New York in the middle of January. I wish he were born in May somewhere.”

In fact, he said, someone has even proposed a May birthday, after the Shear Lock season, which is a sheep-shaving tradition in Ireland.