|666 by topic|
|Ab urbe condita||1419|
|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|
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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.
- Emperor Constans II grants the request of Maurus, bishop 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 Slavs), 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 Yu craft more south-pointing chariot vehicles (a non-magnetic, mechanical-driven directional-compass vehicle that incorporates the use of a differential gear) for Emperor Tenji of Japan, as recorded in the Nihon Shoki.
Events pertaining to religion in the year 666
- Wilfrid returns to 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 (east London) for women.
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- Arnefrit, duke of Friuli (Northern Italy)
- Liu Xiangdao, official of the Tang Dynasty (b. 596)
- Ramla bint Abi Sufyan, a wife of Muhammad
- Yeon Gaesomun, dictator of Goguryeo (b. 603)
- 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