It’s up to you when to start decorating for fall. I have a bad habit of wishing for fall before it comes! But that said, late August is a good time to start. Some feel better by waiting for the true start of the season in September. Anything from mid-August to late September is fine. Around me, people tend to wait until the weather cools off, while others start decorating when school is back in session. Probably the most popular time of all is in October, in anticipation of the Halloween season.