Year 666 (DCLXVI) was a common year starting on Thursday (link will display the full calendar) of the Julian calendar. The denomination 666 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.
|Ab urbe condita||1419|
|Balinese saka calendar||587–588|
|Chinese calendar||乙丑年 (Wood Ox)|
3362 or 3302
— to —
丙寅年 (Fire Tiger)
3363 or 3303
|- Vikram Samvat||722–723|
|- Shaka Samvat||587–588|
|- Kali Yuga||3766–3767|
|Minguo calendar||1246 before ROC|
|Seleucid era||977/978 AG|
|Thai solar calendar||1208–1209|
792 or 411 or −361
— to —
793 or 412 or −360
- Emperor Constans II grants the request of Bishop Maurus of Ravenna, allowing the city to consecrate its bishop without approval from Rome (approximate date).
- Duke Lupus of Friuli revolts against King Grimoald I, with allied Avars. Grimoald takes and devastates Friuli, tracks down Lupus's son Arnefrit (allied with the Slaves), and beats him and kills him in battle at the castle of Nimis. Grimoald appoints Wechtar as the new duke of Friuli.
- Chinese Buddhist monks Zhi Yu and Zhi Yuo craft more south-pointing chariot vehicles (a non-magnetic, mechanical-driven directional-compass vehicle that incorporates the use[clarification needed]
- Wilfrid returns to Great Britain, but is shipwrecked in Sussex. When he finally reaches Northumbria, he finds he has been deposed and is forced to retire to Ripon.
- Earconwald, Anglo-Saxon abbot, establishes the Benedictine abbeys, Chertsey Abbey (Surrey) for men  and Barking Abbey (now in east London) for women.
- Zhang Jiazhen, Chinese official
- John "Reassessing Anglo-Saxon England" pp. 34-35
- Kirby "Earliest English Kings" p. 83
- Yorke "Adaptation of the Anglo-Saxon Royal Courts" Cross Goes North pp. 250-251