There are three options:
- You can park here for free
- You must pay to park here
- You cannot park here
You're examples only relate to the first two, where you can always park, but sometimes you have to pay and sometimes you don't. It's easy in those cases to only specify when you have to pay, even when the schedule is different on Sunday (like your third example).
When you have to add in additional information about when you cannot park (for cleaning, snow removal, altered traffic patterns during heavy commute times, etc), now you need two signs. Well, or two notices on the same sign. You can park for free if it's not a time on either notice. This is in the example at the top of the article.