Tonight when I was watering my trees I noticed my juniper was REALLY root bound. I got it earlier this year at walmart for $3.50. It's still in the black nursery pot. It has roots growing out of the bottom...in fact, it's all roots in there.
I know I need to re-pot it this fall. what I don't know is when in the fall.

If you just want to place it in a new pot and add soil, you can certainly di this now. If yu would like to do more extensive root work on it, my idea would be to either do it now (If you are in temperate climate, northern hemisphere) as most plants will make a last growing spurt in august / september before settling in for winter: The roots are now balanced to the canopy, and if the plant is healthy it should recover easily. Otherwise, I would wait untill earlie spring befor new growth appears.

I'll just slip it in to a bigger pot then This tree is going to need a lot of root work done so I'll wait until spring to do anything else.
Thanks

Spring is indeed the best time for Junipers to get repotted and get a good root-prune!

Good luck when you do re-pot!