The Tachikawa Showa Kinen Park Fireworks Festival will see 5,000 fireworks launched, making it a medium-sized festival by Tokyo standards. About 300,000 people attend. Expect lots of yukata and picnic baskets—it’s a nice idea to bring dinner and make an evening of it. Entrance to the park is entirely free after 6:00pm but if you paid the entrance fee to the park earlier in the day, you’d get a better viewing spot, the organizers say (over 15s: 410 yen; under 15s: 80 yen). Apparently it gets crowded around 5:00pm.
The nearest station is Nishi-Tachikawa or Tachikawa Station on the JR Line.