Review days can only be scheduled at the beginning of a new plan! Once you complete your current plan and sucessfully reach Review Day, tap the flags to progress through your review: open the treasure chest to collect your GoCoins, check out all the cool blocks you collected along your journey, and view your new landmarks and achievement badges. Then manage your still-incomplete tasks, soak in all your progress from your completed tasks, and finally... start planning your next adventure! That's when you'll be able to decide the date of your next Review Day.

See also: Can I change how often Review Day occurs?