You'll probably see St. Agnes Cathedral across town before you actually find this grand building. Pop inside if it's open or just appreciate the facade from outside.
This Rockville Centre principal church is a good place to kick off your day of sightseeing. There's plenty to see close by, including Belmont Park, Nassau Veterans Memorial Coliseum, and Hofstra University. Wander around at your leisure at St. Agnes Cathedral before hitting up these other fine sights.