After logging in, choose the occasional service you want to book - for example a parking space. You then arrive on your personal calendar.
Select the date you want to reserve, then click on "Book".
You will then be notified if your request has match an available service. If it is available, a green light will appear below the date in question. If it is not (yet) available, a yellow light will appear. Updating these indicators may take a few seconds or require a downward swipe to refresh the display.
For more information on all the different light indicators, you can consult this article.