Note: this list is by no means comprehensive, and contains some generalizations based on my own personal experience. If you have a conference I left out to recommend, please do so in the comments!
In no particular order...
BEA ("Book Expo America"): BEA is the largest book trade fair in the United States. It happens every year, in late May/early June. In the past it has been in a few different cities, but it will be at the Javitz Center in New York City through 2015. Those who attend are industry professionals, booksellers, librarians, teachers, authors, aspiring authors, book bloggers, other press people-- you name it and they show up at BEA. Why go? Well, you can get ARCs, go to panels, get books signed, and hang out with your fellow book nerds. ALL WEEK. However, BEA tends to be more "industry" and less friendly than other conferences, as its focus is not on readers but on people who work in the business of publishing.
SCBWI ("Society of Children's Books Writers and Illustrators"): SCBWI is an organization for the writers and illustrators of children's books, which includes all levels of "children's books," including YA. There are a few regional events throughout the year. The summer conference is the biggest one, and it takes place in August in Los Angeles. This is a great conference for writers to connect with each other, learn from each other, and to learn more about the industry we're hoping to be a part of.
Backspace: Backspace is an online writer's organization that hosts the Backspace Writers Conference every year, in New York City, in late May. In my experience, the Backspace conference is best for aspiring authors with polished manuscripts who are ready to get serious about getting an agent/getting published, though I'm sure you can get a lot out of the conference if you're not there yet. (There are a lot of agents in attendance and opportunities to pitch to them! Woo!)
ALA ("American Library Association"): the ALA conference is basically a huge group of librarians coming together in what should be a fantastic combination of awesome. Librarians are the shit. If you want to go, there's the annual conference and the midwinter meeting every year. The annual conference happens around the end of June, and the midwinter meeting is usually in late January. The location changes every time-- the next one will be in Anaheim, California. You can check on the upcoming dates and locations here: http://www.ala.org/conferencesevents/upcoming Not only librarians attend-- there are also authors, book bloggers, and readers in attendance. Why go? Well, you can get ARCs, attend panels that appeal to all readers (not always library-specific), get signed books, and generally experience books in a more relaxed atmosphere than BEA. Authors who attend can discuss their books with librarians, who influence what books their libraries stock and what they recommend to their students-- and they can tell you firsthand what the state of our nation's libraries is. (Hint: not always so well-funded!)
TLA ("Texas Library Association"): TLA is a lot like ALA, but just for Texas librarians. Texas is a great state for the library system, and books that make it on their reading lists are read by practically every student in the state. If you're going to be in Texas in April, and you like books, it's a great place to be.
ABA ("American Booksellers Association"): this conference is everything bookstore and bookseller (people who work in and/or own bookstores) related. The Winter Institute (which is what people are usually referring to when they talk about ABA as a conference) is usually in late February. ABA also rotates-- in 2013 it will be in Kansas City, Missouri. For authors, this is a great opportunity to connect with booksellers who might not otherwise have heard of your book. For booksellers, this is a great opportunity to learn from and network with fellow booksellers and hear about more books to sell. This conference really is just for booksellers, though, so if you aren't one you wouldn't get as much out of it as, say, ALA or even BEA.
RT ("Romantic Times" Booklovers Convention): a convention hosted by RT Book Reviews, an organization that reviews books with a romantic focus (not necessarily just romance, but yes, primarily romance) in a monthly magazine. The location changes every year, but in 2013, the convention will be May 1-May 5 in Kansas City, Missouri. Why go? Well, the usual: ARCs, panels, people watching, the big four-hour-long signing-- this is a lively, fun convention with a lot of variety and a growing YA community. (You may also get to see what romance cover models look like in real life. AND there may be costumes.) Aspiring authors can pick up writing tips and there are a lot of opportunities for self-published authors, too. It can get crazy, though, so try to stay organized! You may get lost.
ComicCon: ComicCon originated as a comic book convention but has now branched out into TV, movies, video games, computer games, books, toys-- basically, if you've got a fandom, they've got something for you. There are a lot of ComicCons throughout the year, but the one most people are referring to when they talk about ComicCon takes place at the end of July in San Diego. (But there's a big list here: http://en.wikipedia.org/wiki/Comic-Con) Why go? Well, there's something for everyone, and reasons for attending range from being a huge comic book fan to just wanting to catch a glimpse of Taylor Lautner. If all you're into is books, you can attend author panels, get ARCs, get things signed, etc. If you don't like maneuvering through dense crowds, though, this might not be the place for you. If you like seeing amazing costumes, it totally is.
NCTE/ALAN ("National Council of Teachers of English"/"Assembly on Literature for Adolescents"): Technically this is two different conferences, but they tend to share the same members and the same time frame. The annual convention moves around, but next year it's in Las Vegas in November. This conference reminded me a lot of ALA, except with English teachers instead of librarians-- there were the same panels that would be interesting to everyone, the same opportunity to connect with people who help shape readers' tastes (and set up author visits!) and to hear about what schools are doing with books.
Y'ALLfest: Y'ALLfest is a young adult book festival in Charleston, South Carolina, in November. The website is here: http://yallfest.org/. This year there will be 44 authors in attendance, and there will be panels, presentations, and signings. I've never been myself, but I hear it's a LOT of fun.
Printers Row Lit Fest: I'm going to quote from this article I found, because it summarizes Printers Row better than I can: "The Printers Row Lit Fest is considered the largest free outdoor literary event in the Midwest with seven stages featuring more than 100 literary programs. The annual festival draws more than 125,000 book lovers to the two-day showcase and attracts more than 200 booksellers from across the country displaying new, used and antiquarian books." All I have to say is: YES PLEASE and also: GO CHICAGO. *cough*
Fandom Specific Conferences: conferences associated with particular genres and fandoms are also great places for writers and authors. I can't get into each one, but hopefully we'll delve more into some of them later this week.
Small, Regional Conferences: my first writer's conference was Midwest Writer's Workshop in Muncie, Indiana. There are plenty of similar, smaller conferences around the country, throughout the year. I used the mighty tool of Google and I recommend that you do, too!
Well, there you have it: conferences and conventions and book festivals, oh my! Go forth and devour books, my friends.