After years of experimenting with this, I now just leave my holiday items in my shop all year round. I've sold many Christmas items in the summer! I generally move them to the end of my shop at the end of the year, but buyers still find them. As far as when to list them, I start as early as September and am still listing holiday items now. I wish you the best, Megan!
(I am a vintage seller so it may be different for handmade...)