January 1st 45 BCE: The Julian calendar came into effect. 42 BCE: The Senate deified Julius Caesar (100-44 BCE). 1766: Death of James Francis Edward Stuart, also known as the 'Old Pretender', the only son of King James II of England, who is interred in St Peter's Basilica. 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 Trapassi, better known as 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 1391: Canonisation of St Bridget of Sweden (c. 1303-73) by Pope Boniface IX (r. 1389-1404). 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 Tosca by Giacomo Puccini took place at the Teatro Costanzi (today’s Teatro dell’ Opera).
January 15th 69 CE: Assassination of Galba (r. 68-69), the fifth emperor of Rome, and the first in the so-called 'Year of the Four Emperors'.
January 16th 27 BCE: The Senate granted Octavian the title Augustus, thereby creating the first Roman Emperor. 1819: Birth of the sculptor Ignazio Jacometti (d. 1883).
January 17th 38 BCE: Marriage of Octavian, the future emperor Augustus (r. 27 BCE - 14 CE), to Livia Drusilla. 1377: Pope Gregory XI (r. 1370-78) made a solemn entrance into Rome, thereby formally returning the seat of the papacy to the Eternal City from Avignon, where it had been based since 1308.
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 Swiss Guards. 1622: Death of Cesare Ripa (b. circa 1555), who penned the highly-influential emblem book L'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. Claudius (r. 41-54) succeeded his nephew as the fourth emperor of Rome.
January 27th 98 CE: Death of the Emperor Nerva (r. 96-98), the first of the so-called ‘Five Good Emperors’. Nerva was succeeded by Trajan. 1866: Death of the Welsh neoclassical sculptor John Gibson (b. 1790).
January 28th 1621: Death of Pope Paul V (r. 1605-21), a member of the Borghese family. Pope Paul V is interred in the Cappella Paolina in the church of Santa Maria Maggiore.
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). 1788: Death of Charles Edward Stuart, also known as Bonnie Prince Charlie or 'the Young Pretender', who is interred in St Peter's Basilica.
January 31st 1629: Death of the architect Carlo Maderno (b. 1556), who is interred in the church of San Giovanni dei Fiorentini.