Some coordinators allow helpers to set their own Cannot Help times in their Preferences. This can be used, for example, if you know that you can never work on Thursday mornings. If, on the other hand, you only want a certain date off (one particular Thursday), then you should use the Time Off section instead of setting repeating Cannot Help Times.
These times are set on your Preferences page.
In the top menu, click Prefs to go to the My Time Preferences section.
Be sure to click the Save button when done.