I know it's only August but the holidays are busy both for selling and personally so if you get prepared well in advance you will save yourself a lot of stress.

Ramp up production. My personal goal is to reach 200 items in my Etsy shop by mid-October. This includes making seasonal items now.
Stock up on supplies. You don't want to run out around the end of November.
If you get priority mail boxes from the USPS they usually have a deadline for ordering more boxes before the holidays. Do it now and you don't have to worry about it. If you ship international stock up on customs forms as well.
Make a list. If you write down all the stuff you need to do before the holiday season starts and get working on it now you're less likely to get overwhelmed.

About the beginning of the holiday season:
While the day after Thanksgiving is the traditional kickoff of holiday shopping plenty of people start earlier. So don't be surprised to see some holiday shoppers earlier than that (especially important if you offer gift wrapping).