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|Ab urbe condita||1058|
|Balinese saka calendar||226–227|
|Chinese calendar||甲子年 (Wood Rat)|
3001 or 2941
— to —
乙丑年 (Wood Ox)
3002 or 2942
|- Vikram Samvat||361–362|
|- Shaka Samvat||226–227|
|- Kali Yuga||3405–3406|
|Iranian calendar||317 BP – 316 BP|
|Islamic calendar||327 BH – 326 BH|
|Minguo calendar||1607 before ROC|
|Seleucid era||616/617 AG|
|Thai solar calendar||847–848|
431 or 50 or −722
— to —
432 or 51 or −721
Year 305 (CCCV) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Constantius and Valerius (or, less frequently, year 1058 Ab urbe condita). The denomination 305 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- May 1 – Emperor Diocletian abdicates at age 60, and retires to his palace at Salona (modern Split) on the Adriatic coast, after a reign of nearly 21 years in which the last vestiges of republican government have disappeared.
- Maximian retires from office and leaves for Campania (southern Italy), to live a life of ease in his luxury villas.
- Constantius Chlorus and Galerius are declared Augusti; Flavius Valerius Severus and Maximinus II Daia are appointed Caesars.
- In the Western Empire, the capital is moved from Rome to Milan. Constantius Chlorus requests leave for his son Constantine I, who remains at Galerius's court in Nicomedia as a virtual hostage.
- Summer – Constantine I joins his father in Gaul, from Bononia (Boulogne); they cross the Channel to Britain and make their way to Eboracum (York), capital of Britannia Secunda and home to a large military base, in order to deal with a rebellion by the Picts.
- Maximinus II (305–313) persecutes the Christians of Egypt, many of whom take refuge in the desert. In time, this refuge leads to the monastic life. In these monasteries Coptic writing develops, supporting the propagation of Christian texts.
- Patron of Pozzuoli, Saint Proculus, and patron of Naples, Saint Januarius are thrown to wild beasts in Pozzuoli's Flavian Amphitheater, then beheaded at Solfatara.
- The Daysan River floods Edessa.
- Landowners dominate the Roman Empire and enjoy the title of senator, which exempts them from the crushing taxes imposed on the rest of the population. The Senate has lost all its power and the landowners almost never attend Senate sessions. Members of municipal senates (curiales or decuriones) are charged with the responsibility of collecting taxes and paying arrears; smaller landowners are held responsible for providing recruits for the Roman army and with keeping wastelands under cultivation.
- Catherine makes a public confession of the Christian gospel at a sacrificial feast ordered by Maximinus II at Alexandria. A virgin of royal descent, she was tortured on a spiked wheel (later called "Catherine Wheel") and beheaded; her remains are spirited to Mount Sinai.
- The Council of Illiberis decrees that priests must be celibate. Additionally, it condemns visiting the homes of Jews and prohibits Christian women from marrying Jews, unless they have converted.
- Saint Damasus, pope (366—383)
- The beheading of Saint Panteleimon the Great Martyr & Healer
- Hierocles, proconsul of Bithynia who instigated the persecution of the Christians under Galerius
- Januarius, patron saint of Naples
- Porphyry, Neoplatonist philosopher
- Proculus of Pozzuoli, patron saint of Pozzuoli
- Saint Catherine of Alexandria, martyr and virgin
- Tuoba Yituo, chieftain of the Tuoba clan (China)
- Wang Rong, politician of the Jin Dynasty (b. 234)