According to Blog Action Day 2007, everyone should publish a post on their blog which relates to an issue of their own choice pertaining to the environment.
They want everyone to post on October 15th.

In addition, everyone should donate the days earnings towards an environmental charity.

Heres their advertisement video:
[youtube=http://www.youtube.com/watch?v=WfO8mGjXoe8]

I plan to participate on October 15th since this is an easy way to contribute your blog towards a good cause.