do you have to cite common knowledge?

No, true common knowledge does not require quoting. Common knowledge is made up of information that is readily available and accepted by a general public and which can be confirmed in different sources without needing expertise.
Examples of what is considered common knowledge would be dates and basic facts such as geography. Since there are different kinds of audiences the border varies and thus it is better to cite every piece of work that the reader might have a reason to doubt. If you are uncertain you should quote and only then check your work for plagiarism.