Now! NO -- yesterday actually. You're behind already! OK. Not really, but my point is that there is no point in waiting. A good personal statement is going to be about something that helped you learn and grow as an individual. While you'll continue to learn and grow throughout the fall of your senior year (even I am still growing -- mainly wider these days) you need time to process the events and experience. So it's unlikely that you'll have enough distance from something that happens in October to write about in November or (erg) December. So start now.
The other thing about personal statements is that they take time. The good ones do at least. It's very hard to throw together a great statement in a two or three drafts on a Saturday in December. A good statement takes time. You need to write it, and then get some distance form it, and return again. It's labor intensive process that should help you learn about yourself and help you to develop as a writer.
So that means you should be starting now.