From the Blog Action web site:
Blog Action Day is an annual nonprofit event that aims to unite the world’s bloggers, podcasters and videocasters, to post about the same issue on the same day. Our aim is to raise awareness and trigger a global discussion. First and last, the purpose of Blog Action Day is to create a discussion. We ask bloggers to take a single day out of their schedule and focus it on an important issue. By doing so on the same day, the blogging community effectively changes the conversation on the web and focuses audiences around the globe on that issue. From the smallest online journals, to huge online magazines, to EU ministers, to professionals and amateurs, Blog Action Day is about mass participation. Anyone is free to join in on Blog Action Day and there is no limit on the number of posts, the type of posts or the direction of thoughts and opinions.