You can start whenever you want, I look at it this way:
People start purchasing for the holidays around August and on. Between June and August people are busy away from their shops going on vacation and enjoying the weather - let's face it, we tend to work a lot less during that time. Suddenly August is here and school starts, and Halloween is here before you know it, and Thanksgiving, and suddenly Christmas is over. Build stock early, less things to stress about between September and December because that time passes way too quick, and there is already too much to think about and do during that time without adding 'stocking shop' to that list! And of course you can (hopefully) start selling your goodies early! :)