Year 784 (DCCLXXXIV) was a leap year starting on Thursday (link will display the full calendar) of the Julian calendar. The denomination 784 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||1537|
|Balinese saka calendar||705–706|
|Chinese calendar||癸亥年 (Water Pig)|
3480 or 3420
— to —
甲子年 (Wood Rat)
3481 or 3421
|- Vikram Samvat||840–841|
|- Shaka Samvat||705–706|
|- Kali Yuga||3884–3885|
|Japanese calendar||Enryaku 3|
|Minguo calendar||1128 before ROC|
|Seleucid era||1095/1096 AG|
|Thai solar calendar||1326–1327|
910 or 529 or −243
— to —
911 or 530 or −242
- Saxon Wars: King Charlemagne begins a campaign in northern Saxony. He ravages Eastphalian territory as far as the Elbe River, while his son, Charles the Younger, defeats a Saxon force in the Lippe Valley. Bad weather hinders Charlemagne's winter campaign in southern Saxony.
- Winter – Charlemagne returns to Eresburg and builds a church, probably on the site of the Irminsul (a pagan religious site). Frankish forces based at Eresburg attack rebel Saxon settlements, and take control of the roads. Charlemagne himself takes part in some of these raids.
- Abd al-Rahman I, Muslim emir of Córdoba (Al-Andalus), begins the construction of the Prayer Hall of the Great Mosque of Córdoba. He uses the mosque (originally called Aljama Mosque) as an adjunct to his palace, and names it in honour of his wife.
- The Japanese begin a war against the Ainu, in the north, on the main island of Honshu. Emperor Kanmu wishes to be free from the influence of the Buddhist monasteries around Nara (then called Heijō), and moves the capital to Nagaoka, ending the Nara period.
- Nagaoka-kyō becomes the Japanese imperial capital.