Wizard magazine [HASHTAG]#176[/HASHTAG] which has a preview of Joss' contribution, describes the motto of the book: Comics celebrities provide art but "they'll be drawing characters they're normally not associated with! For example, 'Buffy The Vampire Slayer' creator Joss Whedon, [...] known for his writing, riffs on Steve Ditko, with a
pin-up featuring Emily the Strange". Wizard mentions July 20-23 (San Diego Comic-Con) as debut of the anniversary issue while Dark Horse's page (see above link) announces that it's going to be released on July 5th.
