As soon as you possibly can, start packing. After you have carefully sorted out the things that you need and the things that you do not need, you can begin to pack your stuff.
It does not matter if you are moving a year from now, a month from now or a week from now. Packing your stuff will give you the peace of mind that you need. For instance, if there are things which you may still need soon, you can label these things and pack them later on but for the things that you do not use often, you can begin stuffing them in boxes.
There is a thin line between packing and sealing. If you’ve packed, it means you’ve gone through your things and you’ve put them in the box. If you have sealed the box then you are good to go. It would be enough to just pack your stuff and seal the box a week or so before the actual moving day that way you can still get the stuff that you may need without destroying the entire sealed box.