Wow... I had never heard of these. I had to search for "blog carnivals" Very interesting!
A blog carnival is a type of blog event. It is similar to a magazine, in that it is dedicated to a particular topic, and is published on a regular schedule, often weekly or monthly. Each edition of a blog carnival is in the form of a blog article that contains permalinks to other blog articles on the particular topic.
Carnival posts are generally collated by the author by soliciting relevant contributions from interested people. The author collects links to these submissions, edits and annotates them and publishes the resulting round-up to his or her blog. Many carnivals have a home page or principal organizer, who lines up guest bloggers to host each edition. This means that the carnival travels, appearing on a different blog each time.
Communities of blog readers, writers, and edition hosts form around specific carnivals. The carnivals provide an aggregation of recent posts by the community on a given topic, and the host provides a level of editing and annotation that helps readers find posts they are interested in. Writers who submit their articles to blog carnivals are rewarded with traffic (if the host decides to give them a link and, perhaps, a positive review).
A recent variant is Synchronised Blogging, or synchroblog, where a group of bloggers agree to post on their own blogs on the same broad topic on the same day. The titles are circulated a day or two beforehand, and each blogger includes links to the other blogs.
A few interesting pages:
And, of course, Blog Carnival.