I came across the word “intercalary” on a post about how writers ride roughshod over cultural naming conventions, though the word has little to do with names. Instead, it refers to days or months added to keep our earthly calendars in sync with the motions of the heavens. Per OED:
Of a day, days, or month: Inserted at intervals in the calendar in order to bring an inexact reckoning of the year into harmony with the solar year.
Leap Day is an intercalation of the lunar calendar. I have to say, though, I’m not quite sure how even my well educated Regency lords are going to wedge this one into a conversation.