if it's pot grown, any time you like. I grow a few pot grown ornamental acers which enjoy net shade from direct sun & burning wind as they can be susceptible. On the the flip side I have two Norway Maples ( acer platanoides ) which grow in the open garden & totally impervious to our harsh Fenland winters & direct sun etc.
Having said that I see superb mature acer varieties flourishing in exposed situations & doing really well. They are beautiful in all their varieties.