That's the average amount of days they say it takes to break a habit or form a new one. This indicates to me that this is how long it takes to convince your subconscious of something. I had a class in college where the entire premise was that if you convince yourself of something, your subconscious will strive to make it happen. I've heard a similar concept from other places as well.

I'm not a psychologist, but this indicates to me that if you were to form a new habit by doing this for 66 days, you will convince your subconscious of it, which will in turn cause it to manifest into reality ie: you can train yourself to be more positive