For festivals in Hiroshima, in may they have the beautiful Hiroshima Flower Festival in May:
Hiroshima Flower Festival
A major event during Golden Week alive with flower, green and music.
Wed. May 03-Fri. May 05
Peace Boulevard (Heiwa Odori), Peace MemorialPark
In July they have the fireworks festival:
Fireworks Display at Hiroshima Port
(Hiroshima Minato Yume Hanabi Taikai)
About 10,000 fireworks such as bottle rockets and water fireworks will color night sky beautifully.
Sat. Jul. 22
Hiroshima Port 10,000 ton class berth
As for festivals in Osaka:
In may they have:
Harvest Hill, Strawberry Picking
Enjoy three varieties of strawberries in all-you-can-eat style, with no time limit.
PLACE : Harvest Hill
DATE : Tuesday, Thursday, Saturday and Sunday of January 2(Tue) - May 31(Thu), 2018 11:00- *Reservation is required
You can see more here: http://www.osaka-info.jp/en/events/festivals_events/post_207.html
I hope this helps!