SAN FRANCISCO — A steady stream of Darth Vaders, Princess Leias, Boba Fetts and, of course, Stormtroopers descended on AT&T Park here Sunday as the world-champion San Francisco Giants played host to the Empire for Star Wars Day.

In a city known for its geek fascinations, seeing Jedi mix with hard-core baseball fans is not an uncommon occurrence, but the Giants' game against the Arizona Diamondbacks was the first-ever Star Wars Day for the franchise.

And the team went all-out. Fans got a pre-game costume contest, a post-game screening of Star Wars Episode V: The Empire Strikes Back in the...