Team Challenges integrate with Coach. Once a member opts into a challenge, that challenge becomes their new goal in Coach and those workouts will be available on the days they have selected. If the member does not take the challenge workout on the day assigned, it will be pushed to the following day.
If you need to change the days you've selected, go to Coach and click on "Edit" and press and hold an activity to move it. Or, click it and select "Move".