I usually start listing my autumn items mid-July to August 1 (which means I need to start making them in the next couple of weeks - yikes!), and Christmas items mid-September to October 1. The key is to get them out well in advance of the actual season because treasury curators, Etsy admin, and bloggers will start running those themes super early. The earlier you have your products listed, the longer your period of visibility for features and for buyers who are only browsing in September, but may see you over and over again from September to November, and come back to you when it's time to finally start making their holiday purchases in November and December.