Circa means “about” or “around” when we give a date we cannot pin to one exact day or year: circa 1920, circa 300 BCE. In print it is often shortened to c. or ca. (c. 1920). It is a learned borrowing from Latin, so it belongs on museum labels, in history books, catalogues, and careful essays — not in a text about when a friend will arrive. You put it before a year, a decade, or a century. It does not change form, and it does not need of. Though actually, it is not a fancy synonym of about in every sentence. “I’ll be there circa six” sounds stiff, or like a joke pretending to be a historian. Save it for the past when the exact year has been lost, or was never written down. Some very formal lists also put it before a number of objects in an old account, but dates are the use students keep meeting.