How the planner works
The gap comes from a rule of thumb in a review of learning techniques by Dunlosky and colleagues (2013): in the spacing studies they summarize, performance was best when the gap between study sessions was about 10 to 20% of the time the material had to be remembered. Here that time is the number of days from your first study session to the test.
The planner shows that 10 to 20% window, then lists example review dates using a gap in the middle of it (15%), repeated until the test.
Note: The studies behind the rule tested one gap between two sessions. Repeating the gap for several reviews is an extension of that finding, not a schedule that was tested as a whole. Treat the dates as a starting point and move them to fit your week.
Each review works best as a self-test rather than a reread: close the book and write or say what you remember, then check. The guide to the spaced repetition schedule explains the research and the limits in full.