From the Sherlock Holmes Wikipedia page [which is an awesome resource/rabbit hole for info on its own]:
Though not the first fictional detective, Sherlock Holmes is arguably the best known, with Guinness World Records listing him as the "most portrayed movie character" in history. Holmes's popularity and fame are such that many have believed him to be not a fictional character but a real individual; numerous literary and fan societies have been founded that pretend to operate on this principle. Widely considered a British cultural icon, the character and stories have had a profound and lasting effect on mystery writing and popular culture as a whole, with the original tales as well as thousands written by authors other than Conan Doyle being adapted into stage and radio plays, television, films, video games, and other media for over one hundred years.But, Sherlock Holmes is more than just popular, people all over the world, from all backgrounds have not only been drawn to him, but they have made him their own. And for many, Holmes is more than a fictional character, he is a real person.
A few years ago I had a chance to go to 221b Baker Street and visit the “world’s most famous address.” Holmes’ “home" is a museum now, and on the top floor they keep a binder with letters that real people have written to Holmes. The museum constantly rotates the letters so that the most recent are there for you to look at [they keep them all in storage and they have hundreds of thousands which have been sent to the address since the 1800s]. Seeing those letters, people asking for help with a mystery, telling Holmes what he means to them, and just expressing thanks was not only moving, but educational.
Seeing how much this fictional character meant to so many people really drove home the theories I teach. Doyle’s Holmes was so vividly drawn that he is literally a friend to people, people who could never meet him. That is intense appeal personified. I was blown away by how a character, made up by a man, brought to life on the page, over a century ago could be so real. Even I could feel him when I was there. When people have strong feelings about any book [good or bad] I remind myself of that visit to 221b Baker Street and remember how fake things can become real. I will never forget that day and I am simply someone who likes Holmes, not an uber fan in any way [now my 15 year old daughter, she is in the uber fan category].
You can celebrate Sherlock Holmes anytime of year in the library-- he is that popular and you have that many items to make a display in each department - but today you can take advantage of the intense coverage like that over at Book Riot which has links, lists, and more from just about every conceivable angle. It will help you celebrate a little better.
I also highly suggest Leslie Klinger’s award winning work writing about Holmes and annotating his stories.
Happy Sherlock Holmes Day!