Suppose you accidentally clicked off Wednesday.
You do not see this day anymore, because you have set in the Eye-function of your calendar that you want to hide unavailable days.
1. Click the eye at the top right of your calendar view
2. Then click on ''Unavailable days'' in the pop-up screen to show the non-schedulable days in your calendar.
3. If you have enabled the unavailable days, you will see them (greyed out) in your planning board.
4. Then move your mouse to the unavailable day and click on the slider to the left of the day name.
Finally, you can hide the unavailable days again via the Eye.