Evolution Day observes the date of the original publication of Charles Darwin's On the Origin of Species in 1859. The book can be read online here or here or here. Some people continue to misunderstand the theory, but there are explanations online that make it easy to understand even for folks like me for whom science is not a first language. LiveScience.com has some information, How Stuff Works has some basics, and Wikipedia has an entry.
This is a history of the theory:
This is a Nova documentary:
I haven't seen the videos yet, but I plan to watch them later in observance of the day.