Free Comic Book Day is a single day – the first Saturday in May each year – when participating comic book shops across North America and around the world give away comic books absolutely FREE* to anyone who comes into their stores. *Check with your local shop for their participation and rules.
The way it works is that each comic book publisher creates special comics for the giveaway, then the stores that are participating buy them to give out to their customers. The nominal fee covers printing and shipping.  The comics are a great cross section of what comics have to offer from almost every publisher with books directly aimed at kids and others for everyone. The whole idea is to get people reading comics.
We’ve gone out for the last 6 and had a ball. It is great fun to watch the little kids with free comics, getting impatient with Mom or Dad so they can go read. It is also really cool to see the stores packed full of people.
UPDATE:
I just got an advance look at the Archaia Entertainment giveaway. Holy Awesomeness Batman!!! This is the coolest giveaway ever, It’s a 9 x 6 hardcover with stories from Mouse Guard, Labyrinth, Return of The Dappermen, Rust, Cursed Pirate Girl and our new favorite COW BOY!!!  If you miss out on this book you are CRAZY. It is a perfect way to introduce people to these great comics and it was put together with a lot of love for fans and future fans.
A lot of stores do events as a part of it so check near by stores to see if they have creators coming in to sign or do sketches. You can check here:
http://www.freecomicbookday.com/
Here’s some of the books being given away this year:
http://www.freecomicbookday.com/Home/1/1/27/981