Yes, keep it outside for a couple of weeks. If it is likely to snow you might want to cover the pot with some type of cloth or something to prevent freezing.

I would suggest planting it when the first flowering trees begin to bloom. This should be just before it starts to leaf out, but after the temperatures have risen enough for the ground to thaw. (In reality I think fall is the best time for planting, but you might as well go ahead and plant it this spring)

If I may I'd like to make a couple of suggestions. If you are not aware, silver maples are notorious for becoming a major problem in the landscape. The wood is very weak and the limbs often split and fall on things. If I were you I would be sure to plant this tree away from any structures or near anything that you'd prefer not to get crushed by falling debree.

It is also important that you exhibit good pruning habits with these trees. Be sure to pick a central leader and prune limbs that grow at nearly verticle angles to prevent crotch breakage.