If you are doing little or no root work you could do it now. On the other hand if it has been in the pot for a few years and the walls of the pot are lined with roots it probably needs to be root pruned first. In that case next spring would be the ideal time to prune the roots and plant it.

Actually that's a bad idea, cutting the roots before planting will slow down the growth for the first season and what he's/she's going for here is growth to thicken the trunk. Always better done in the ground anyway.
Simply spread the roots out, as you would do when planting something from a nursery.
Given the time of year you can still get 2.5 to 3 months of growth this season before it goes dormant.

Bob