The Kobe fireworks display is held on the first Saturday of August every year. This is the same day as the bigger Yodogawa fireworks display in Osaka, but Kobe is much more photogenic so it's worth making the trip. If you are planning on going next year, I'd recommend avoiding the crowds at Meriken park and joining... the crowds on port island. You'll miss out on the cheesy music to which the fireworks are choreographed, but you'll gain the impressive backdrop of Harborland and Mount Rokko.
Sunset before the fireworks.
I'd also recommend stopping by the Lucy curry house in Motomachi for dinner; their ostukare is incredible.