the Kamakura fireworks festival takes place every second tuesday of august, that means, yes, the next year will be held on august 12th.
I suggest you really consider going to Fushimi Inari. It's close to Kyoto, and it's stunning!! It's a mountain path covered by thousands of red torii.
รจ uno scenario stupendo, assolutamente da vedere^^ te lo consiglio indubbiamente!
Link:
Kyoto Travel: Fushimi Inari Shrine (Fushimi Inari Taisha)
About Nikko, it can be wisited without major problems in one day, but don't go just to the temple area. Take a bus and go up the mountain to Kegon falls and Chuzenji lake!