#ArchiveDay happens every Saturday, all day.
It's open to anyone with a blog; all you need to do is tweet a link to an old post, using the hashtag #ArchiveDay.
Then follow the tag through the day, and re-tweet others with that tag.
The idea is that you also read other blogs, and that other tweeters read your blog. You never know; you might find a blog you want to follow in future - and some Twitter friends to boot!