You can harvest the seeds when the leaves have turned red according to the New England Wilflower Society guide to trees and shrubs. They have to be cold stratified.

Thanks for your speedy and helpful reply.

I harvested some this year and also plucked them when the berries were blue. Inside the blue berry, in a few, was a hard seed. Some of the berries had soft seeds that I could easily crush. I took these hard seeds and dropped them in water. Only a few sank, so those are what I saved to germinate. The fatter berries had the better seeds.