Q: Why do September, October, November, and December come from Latin numbers, but the rest of the months aren’t numerical?
A: We inherited the names for our months from the Romans, who used numerals for some and other designations for the rest. In fact, the Romans sometimes went back and forth, switching from a number to another term and vice versa.
For example, mēnsis Quintilis (“fifth month”) was renamed mēnsis Iulius (“month of Julius”) for Julius Caesar, while mēnsis Sextilis (“sixth month”) was renamed mēnsis Augustus for Augustus Caesar, as Matthew Bunson notes in the Encyclopedia of the Roman Empire (2002).
Caligula changed mēnsis September to mēnsis Germanicus to honor his father, according to the Roman historian Suetonius, but the month went back to its numerical name after the emperor’s death.
And Nero renamed several months, including mēnsis Neroneus for mēnsis Aprīlis, but again, the new names didn’t stick, Suetonius writes in De Vita Caesarum (“On the Lives of the Caesars”).
You didn’t ask, but some readers are probably wondering why September, October, November, and December are our ninth, tenth, eleventh, and twelfth months, while the words mean seventh, eighth, ninth, and tenth in Latin.
As the Oxford English Dictionary explains, the “ancient Roman calendar (dating from around the mid 8th cent. B.C.) had ten months.”
The original months were Mārtius, Aprīlis, Māius, Iūnius, Quintīlis, Sextīlis, September, Octōber, November, and December.
Around the year 713 BC, according to the OED, Iānuārius and Februārius were added to the end.
But in 153 BC, the dictionary says, “the beginning of the year was moved to 1 January, when the Roman consuls were elected,” throwing the original meanings of the numerical months out of sync with the calendar.
“This new ordering of the months remained when the Julian calendar was introduced in 45 B.C. and in the Gregorian calendar widely used today,” the OED adds.
As for those non-numerical months, Mārtius was named for Mars, the Roman god of war, according to the OED. Aprīlis is “of uncertain origin; perhaps [from] Etruscan.” Māius was named for Maia, the ancient Roman goddess of fertility and spring, and Iūnius for Juno, the goddess of marriage.
Of the two later additions, Iānuārius was named for Janus, the god of beginnings, while Februārius comes from februa, Latin for means of purification (the Roman festival of purification was held on the fifteenth day of February).