Yukata Festival in Hiroshima
Hiroshima city's Tokasan (Yukata) Festival
Tokasan festival is one of the big festivals of the year in the Hiroshima city center. Food, drink and game stalls to enjoy as well as a dancing area to take part in traditional Japanese "obon" style "odori" dancing around the central drum and music tower.
Tokasan is held on the first weekend in June every year in Hiroshima's city center.
My favorite thing about this festival is that it is the start of the Yukata (summer kimono) season which makes for a colorful, fun crowd to be a part of. You can buy a set of yukata-robe/obi-belt/geta-shoes for under ¥10,000 in many shops on the main Hondori shopping street: http://www.gethiroshima.com/lifestyle/shopping/shopping-in-hondori/
Yukata sales start from mid-May and continue throughout the summer. I would also recommend picking up a straw hat and nice fan to go with your Yukata- it is often very humid at summer festivals in Japan. If it is especially warm, follow the lead of locals and drape a light face towel around your neck to wipe away sweat. It is not easy to put on a Yukata for the first time, so take video or pictures of the process as you try it on in the shop (with an assistant's help of course) or even arrange to go back to the shop on the day you want to wear it to have them help fit it properly. A lot of shops include fitting for the festival in the sale price. Alternatively, you can pay a mere ¥500 to have it professionally adjusted in "Shareo" the underground shopping mall's central market area where there will be a special stall on the days of the Tokasan festival.
See GetHiroshima.com: http://www.gethiroshima.com for updates on the festival in the year you are planning to go as the dates and entertainment changes a little every year.