How do I know which days to workout?

That's up to you! Vi's training plans will not assign specific days to specific workouts but will provide you with the workouts you should complete that week. Vi knows that your schedule can change at any time, so she's always ready to run on the days that work best for you! The number of workouts she gives you is based on the number of days you told her you wanted to run. 

Once you've done a workout, it will be marked as completed.