It's not just to celebrate the ability to read, but more so the freedom to read whatever it is you want to read - without fear of criticism, descrimination or even punishment. That we can just walk into a library and borrow a book on any number of topics - sexuality, art, history, medicine - is something we often take for granted in Canada, but in many countries of the world that freedom just doesn't exist. Even if the books are available, access to them may be limited or even monitored. Even in post 9/11 America with the Patriot Act, libraries were (and even still may be) monitored by the government and required to hand over patron records if there was suspicion of terrorist activity.
Libraries can be said to be equalizers. It doesn't matter how much money you make or what kind of vehicle you drive (if any). It doesn't matter if you are conservative or liberal, if you believe in same-sex marriage or not, are pro-choice or pro-life. When you walk through the doors of a library, everyone has the same priviledges. That's why it's so important that libraries continue to uphold the principles of the Canadian Library Association's Statement on Intellectual Freedom which reads in part, "It is the responsibility of libraries to guarantee and facilitate access to all expressions of knowledge and intellectual activity, including those which some elements of society may consider to be unconventional, unpopular or unacceptable. To this end, libraries shall acquire and make available the widest variety of materials."
To consciously not do the above is censorship. So is not ordering that copy of Fifty Shades of Grey because you think it's "filth" despite having patrons repeatedly ask for it.
In my collection development class we discussed book selection, and that for every book you choose, there's a book you didn't chose -which could be argued to be censorship. However, as long as you are keeping in mind the diversity of your patrons, the popularity of currently literature, the guidance of your collection development policy, use selection tools such as reviews and recommendations to make choices and refrain from making decisions based on your own personal bias (yes, you do have one), you are doing your job as a librarian.
To help libraries celebrate the Freedom to Read, the Book and Periodical Council (BPC) has created the Freedom to Read website and sponsor the annual celebration. On the website, you'll find an amazing amount of resources: ideas on how to celebrate (quizzes, a banned-book book club, book display suggestions, etc), censorship in Canada (including lists of books which have been challenged in our country - is your favourite on the list? You might be surprised...), an event listing where you can submit yours for all to see, and Links and Resources that provide information on a variety of things, such as what to do with a challenge, the history of censorship and more.
If you're looking for some last minute ideas to help you get involved with Freedom to Read at your library, I suggest a quick search on pinterest using the search term "banned books". You'll get great results showing what other libraries are doing, such as: book displays, book lists, art work and activities.
In the United States, an equivalent celebration is held by the American Library Association every September called Banned Book Week. The Banned Book Week website has a lot of great resources as well, including lists of frequently challenged books by decade, century and author.