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Year 799 (DCCXCIX) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar. The denomination 799 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||1552|
|Balinese saka calendar||720–721|
|Chinese calendar||戊寅年 (Earth Tiger)|
3495 or 3435
— to —
己卯年 (Earth Rabbit)
3496 or 3436
|- Vikram Samvat||855–856|
|- Shaka Samvat||720–721|
|- Kali Yuga||3899–3900|
|Japanese calendar||Enryaku 18|
|Minguo calendar||1113 before ROC|
|Seleucid era||1110/1111 AG|
|Thai solar calendar||1341–1342|
925 or 544 or −228
— to —
926 or 545 or −227
- Autumn – Siege of Trsat: Višeslav, prince or duke of Dalmatian Croatia, decisively defeats an invading Frankish army under Eric of Friuli, during the siege at the fortress city of Trsat (Rijeka).
- King Eardwulf of Northumbria, worried about further rivals, has ealdorman Moll killed. Former king Osbald dies as an abbot in exile. He is buried in an unmarked grave in York Minster.
- April 25 – Pope Leo III is physically attacked by a band of aristocratic conspirators, under the leadership of a public official who is a nephew of the late Pope Adrian I. After mistreatment and attempted disfigurement by the citizens of Rome, Leo flees to the court of King Charlemagne at Paderborn (modern Germany) to seek protection. He sends him back with Frankish agents, and restores him to the papal throne.