If you ever have questions about what is and is not common knowledge, ask your professor, the Writing Center, or a librarian.
You don't have to cite something that is considered common knowledge. HOWEVER. It is important to remember that what is considered "common knowledge" varies depending on the type of assignment, the context in which you are writing, and who your audience is. Additionally, it's important to note that even though you know something, that doesn't necessarily make it common knowledge.
Good rule of thumb: If you can find it uncited in 5+ sources, it's probably common knowledge.
Conversely, consider these very specific examples that are NOT common knowledge and DO need to be cited: