January 1st 45 BCE: The Julian calendar came into effect. 42 BCE: The Senate deified Julius Caesar (100-44 BCE). January 2nd 533: Pope John II (r. 533-35) became the first pontiff to take a new name on election, regarding his birth name Mercurius to be an inappropriate name for a pope.
January 3rd 1521: Pope Leo X (r. 1513-21) issued the papal bull Decet Romanum Pontificem, which excommunicated Martin Luther. 1698: Birth of Pietro Metastasio, poet and librettist.
January 4th 1975: Death of Carlo Levi (b. 1902), author of the book Christo si è fermato a Eboli (Christ Stopped at Eboli).
January 6th 1536: Death of the painter and architect Baldassare Peruzzi (b. 1481), who is buried in the Pantheon.
January 7th 1566: Election of Pope Pius V (r. 1566-72). 1655: Death of Pope Innocent X (r. 1644-55), a member of the Pamphilj family, who is interred in the church of Sant' Agnese in Agone.
January 8th 1616: Birth of the architect Giovanni Antonio de’ Rossi (d. 1695). 1713: Death of the Baroque composer Arcangelo Corelli (b. 1653), who is buried in the Pantheon.
January 9th 1522: Election of Adrian VI (r. 1522-23), who would be the last non-Italian pope for the next 450 years. 1878: Death of Victor Emmanuel II (r. 1861-78), the first king of Italy, who is buried in the Pantheon.
January 12th 1674: Death of the composer Giacomo Carissimi (b. 1605).
January 13th 86 BCE: Death of the general and politician Gaius Marius (b. 157 BCE).
January 14th 83 BCE: Birth of the general and politician Marcus Antonius, who is known in English as Mark Antony. 1506: The ancient sculpture of Laocoonwas unearthed in a vineyard on the Esquiline Hill. 1900: The premiere of the opera Tosca by Giacomo Puccini took place at the Teatro Costanzi (today’s Teatro dell’ Opera).
January 16th 27 BCE: The Senate granted Octavian the title Augustus, thus creating the first Roman Emperor. 1819: Birth of the sculptor Ignazio Jacometti (d. 1883).
January 17th 1377: The French Pope Gregory XI (r. 1370-78) made a solemn entrance into the Eternal City, thereby formally returning the Curia from Avignon to Rome.
January 18th 1987: Death of the painter Renato Guttoso (b. 1911).
January 20th 1922: Death of Pope Benedict XV (r. 1914-22).
January 22nd 1506: Foundation of the Papal Swiss Guard. 1622: Death of the writer Cesare Ripa, who penned the much-influential Iconologia (1593). 1922: Death of Pope Benedict XV (r. 1914-22). January 23rd 1752: Birth of Muzio Clementi (d. 1832), pianist and composer. January 24th 41 CE: Assassination of the emperor Caligula (r. 37-41) by officers of the Praetorian Guard.
January 30th 59 BCE: Birth of Livia Drusilla (d. 29 CE), who, as the wife of Augustus, reigned as Empress of Rome from 27 BCE to 14 CE. 1592: Election of Pope Clement VIII (r. 1592-1605). 1628: Death of the architect Carlo Maderno (b. 1556), who is interred in the church of San Giovanni dei Fiorentini. 1788: Death of Charles Edward Stuart, also known as Bonnie Prince Charlie or 'the Young Pretender', who is interred in St Peter's Basilica.