Fatsia Japonica isn’t hardy here in USDA zone 5 so, if you plant it outside now, you’ll need to dig it out and pot it up to be brought inside before frost in the fall or lose it.  Perhaps repot it into a larger pot now to spend the summer outside that way.  You could sink that larger pot into the soil for the summer and that would make it easier to bring inside at the end of the growing season.  It is warm enough now (5/20/16) for it to go outside without any danger.