|Ab urbe condita||1274|
|Balinese saka calendar||442–443|
|Chinese calendar||庚子年 (Metal Rat)|
3217 or 3157
— to —
辛丑年 (Metal Ox)
3218 or 3158
|- Vikram Samvat||577–578|
|- Shaka Samvat||442–443|
|- Kali Yuga||3621–3622|
|Iranian calendar||101 BP – 100 BP|
|Islamic calendar||104 BH – 103 BH|
|Minguo calendar||1391 before ROC|
|Seleucid era||832/833 AG|
|Thai solar calendar||1063–1064|
647 or 266 or −506
— to —
648 or 267 or −505
Year 521 (DXXI) was a common year starting on Friday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Sabbatius and Valerius (or, less frequently, year 1274 Ab urbe condita). The denomination 521 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.
- Future Byzantine emperor Justinian, age 39, is appointed consul. He later becomes Commander-in-chief of the army of the East.
- Ma`adikarib Ya`fur becomes king, supported by the Aksumites; he begins a military campaign against the Arabian tribes.
- February 22 – Samson of Dol is ordained as bishop in Brittany, on the Feast of the Chair of Saint Peter.
- Ecclesius becomes a bishop of Ravenna.