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|321 by topic|
|Ab urbe condita||1074|
|Balinese saka calendar||242–243|
|Chinese calendar||庚辰年 (Metal Dragon)|
3017 or 2957
— to —
辛巳年 (Metal Snake)
3018 or 2958
|- Vikram Samvat||377–378|
|- Shaka Samvat||242–243|
|- Kali Yuga||3421–3422|
|Iranian calendar||301 BP – 300 BP|
|Islamic calendar||310 BH – 309 BH|
|Minguo calendar||1591 before ROC|
|Seleucid era||632/633 AG|
|Thai solar calendar||863–864|
447 or 66 or −706
— to —
448 or 67 or −705
Year 321 (CCCXXI) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Crispus and Constantinus (or, less frequently, year 1074 Ab urbe condita). The denomination 321 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.
- Emperor Constantine I expels the Goths from the Danube frontier and repairs Trajan's Bridge. He leads an expedition into the old province Dacia (modern Romania) and makes peace with the barbarians.
- March 7 - Constantine I signs legislation directing urban residents to refrain from work, and businesses to be closed, on the "venerable day of the Sun". An exception is made for agriculture.
Arts and sciences
Food and drink
- Constantine I assigns convicts to grind Rome's flour, in a move to hold back the rising price of food in an empire whose population has shrunk as a result of plague (see 309 AD).
- The Christian Church is allowed to hold property.
- A synod held in Alexandria condemns Arianism.
- Jews in today's Germany are documented for the first time, in Colonia Agrippinensium (Cologne).
- July 3 – Valentinian I, Roman emperor (d. 375)
- Du Lingyang, empress of the Jin dynasty (d. 341)
- Jin Chengdi, emperor of the Jin dynasty (d. 342)