I need juniper berries for a sauerbraten recipe, but they are not easy to find at the local Megamart. The bit of looking I've done says that they are ripe when they are blue to purple, and people seem to harvest them in September/October. It apparently takes three years to get to the ripe stage, and plants will have a mixture of berries in various stages of development.
You need to pick them from the right species of plant. A few are poisonous and a lot others are nasty tasting. I don't know enough to know the difference between species, so I'm still looking locally for some from a store, but I just might have to order some from an on-line spice house.
Once source I found for info:
http://www.nytimes.com/2008/03/20/garden/20qna.html?_r=0