Expect temperatures to hover in the high 30s, a number that can drop quickly as night comes on. They might set a mountain on fire, but you won't be close enough to feel the warmth (and if you are, you've got bigger things to worry about).
A crowded burn
The Wakakusa Yamayaki regularly attracts crowds in excess of 100,000. While many are day-tripping in from other parts of Japan, you'll want to book accommodation well in advance. If you intend to watch the procession or rice cracker tossing, arrive early.
Cut into Kudzu
Kudzu, the annoying weed that is bane of many a gardener, is a local culinary specialty in these parts. Try the local kuzu-kiri (noodles made out of kudzu).
If the weather is particularly sodden, the festival date may be moved.