Twentieth Century Fox and James Cameron have declared Aug. 21 “Avatar Day,” announcing at Comic Con yesterday that free screenings of a 15-minute preview of Cameron’s long-awaited sci-fi epic unveiled at Comic Con will be offered at 3-D venues around the world on that date. I’d venture a guess that little piece of news has Harvey Weinstein reaching for his Maalox and Quentin Tarantino his machete, since they were both doubtless assuming fanboys would focus their excitement around their World War II epic “Inglourious Basterds,” which is bowing that same weekend in the U.S. and much of the world.