Years ago, we visited the redwoods forest in early June. It didn't rain during our hikes.
This web link explains; https://www.nps.gov/redw/planyourvisit/weather.htm
Oceanic influences ensure fairly constant year-round temperatures along California's redwood coast: mid-40s to low-60s°F. Winters are cool with considerable precipitation. From October through April, a high pressure area sitting atop the North Pacific drives a series of storms onshore, dumping the majority 60-80 inches of annual rain over the region.
I'd suggest the later you go the better.
We always planned for rain during our hikes. I got to carry the day backpack with the rain gear, first aid, food, etc,.. If it rained we were prepared for that possibility.
Hiking there was awesome. Those are some HUGE trees.
Have a good time and enjoy the trees.