One larger and somewhat famous fireworks festival in Japan happens every year in Fukuroi city, Shizuoka prefecture. They put on quite the show, and is certainly worth viewing with the thousands of blasts they shoot off that night. Rather than tell you about it though, I've included pictures below. Scroll down for information about this festival and how to get there.
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When: Every year - typically on the weekend of the second week of August. Start time is of course when the sun sets, around 7 p.m. Goes until 9 p.m.
How to get there: Go to Aino Station (愛野) if coming from the direction of Tokyo. If coming from the direction of Osaka, then go to Fukuroi Station (袋井). There are many places to view the fireworks, including some paid seating areas, but I would recommend finding seats at Aino Park (愛野公園). Seats at the nearby river are also good. Aino Park is a 26 minute walk from both Fukuroi and Aino stations.
*Remember to bring a tarp and/or tent if hanging out all day. It's best to get there as early possible to claim a place to sit. At least arrive two hours early - otherwise you may be out of luck.
*Note on leaving: Leave before the finish to beat the crowds to the station if taking the train. Otherwise, be prepared to wait at the train station for a half hour or more just leave.
Cost: Free. Bring food or money for food.
Bathrooms: Yes. Bring toilet paper or tissue and hand sanitizer as the bathrooms will run out of toilet paper.
Japanese required: None, unless buying food.
Official Website (in Japanese): http://www.fukuroi-hanabi.net/