Compared with big cities fireworks festival, duration of the festival time is shorter.
BUT this one is less crowded and enough space to watch :)
And this festival has history about 400years, that's why you will see lanterns floating in the sea and boats which decorated specially to send off soul of dead people of the year( you don't need to be scare). So this is a traditional festival not like fireworks festivals which you can see most of big cities in Japan.
From 19:00 Official time for let lantern flow on the sea. (If you would like to join, around 1000yen for white&red lantern, usually you get them as a pair.) Usually locals let the lantern flow from around 17:00.
From 19:30-20:30 Fireworks festival
From 20:30- Bon Odori-dance(Japanese traditional dance) of Miyazu, anyone can participate.
Mainly, people watch at Shimasaki Park(島崎公園) where is about 10mins walk from Miyazu station.