The names of Roman months originally functioned as adjectives (e.g., the January kalends occur in the January month) before being treated as substantive nouns in their own right (e.g., the kalends of January occur in January). Some of their etymologies are well-established: January and March honor the gods Janus and Mars; July and August honor Julius Caesar and his successor, the emperor Augustus; and the months Quintilis, Sextilis, September, October, November, and December are archaic adjectives formed from the ordinal numbers from , their position in the calendar when it began around the spring equinox in March. Others are uncertain. February may derive from the Februa festival or its eponymous ' ("purifications, expiatory offerings"), whose name may be either Sabine or preserve an archaic word for sulphuric. April may relate to the Etruscan goddess Apru or the verb ' ("to open"). May and June may honor Maia and Juno or derive from archaic terms for "senior" and "junior". A few emperors attempted to add themselves to the calendar after Augustus, but without enduring success.