I have to disagree, "No parking" is the rule. "Except on Sundays" is clearly an exception, that's why it uses the word "except". Sure, you can define it as part of the rule, but then the concept of "exceptions" loses all meaning.
I think part of the problem may be that you are approaching this from the standpoint of mathematics, whereas the phrase itself comes from the field of law and social science (which would be more applicable to behavior on a forum). The exception, "you may park here on Sundays", proves that a rule exists governing parking in this particular spot depending upon the day of the week, even if you didn't know what the actual rule is.