Christmas will be too late, as the fleece will fall off in the processing. Depending on where you are (climate), about early November is the last time for taking the skin off. Collect it from the abattoir asap after slaughter, so you can get it home and covered with salt within 2 hours of slaughter. Don't be surprised at the 'purse' dangling there. Your lamb will be pretty small at that time, but if it's just for home consumption, and the reason for slaughter is to get a good skin, then it's no problem.
I've never tanned a sheep skin myself - way too much work to do all that pummeling and stretching. My boys did try tanning a grey squirrel and it turned into a Frisbee.
We'll need feedback on how you get on.