If you find a mistake in my translation, please let me know. Anno vero MM, more consueto dies bissextus intercaletur, februario dies XXIX continente, idemque ordo intermittendi intercalandique bissextum diem in quadringentis quibusque annis perpetuo conservetur. Moreover, in the year 2000, according to the usual observance, a bissextile day is intercalated, with February having 29 days, and the same rule for interposing and intercalating a bissextile day in the four-hundredth years is observed in every year forever. A bissextile year is a year containing a bissextile day. I don't know whether the original Gregorian calendar kept the intercalary day on the 24th of February or moved it to the 29th.
Intercalation is the insertion of something, traditionally as of a leap day into a calendar.