Why eighty-four
Twenty-one days is a slogan. Sixty-six is the median.
Every planner in the category prints "21 days to build a habit." The published research found a median of 66 days to automaticity, with a range of 18 to 254 (Lally et al., 2010). Eighty-four clears the median — a length that matches the published evidence.
We won't promise you a formed habit. The honest promise is the one written in the book: about a minute a day, ten minutes once a week — and at the end, evidence, in your own handwriting, of who does the work.
“One missed day is weather. Two in a row is a decision.”