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|AD 420 by topic|
|Ab urbe condita||1173|
|Balinese saka calendar||341–342|
|Chinese calendar||己未年 (Earth Goat)|
3116 or 3056
— to —
庚申年 (Metal Monkey)
3117 or 3057
|- Vikram Samvat||476–477|
|- Shaka Samvat||341–342|
|- Kali Yuga||3520–3521|
|Iranian calendar||202 BP – 201 BP|
|Islamic calendar||208 BH – 207 BH|
|Minguo calendar||1492 before ROC|
|Seleucid era||731/732 AG|
|Thai solar calendar||962–963|
546 or 165 or −607
— to —
547 or 166 or −606
Year 420 (CDXX) was a leap year starting on Thursday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Theodosius and Constantius (or, less frequently, year 1173 Ab urbe condita). The denomination 420 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.
- The legendary Pharamond is said to have led the Franks across the Rhine.
- The Huns, under leadership of the brothers Octar and Rugila, expand their rule through neighbouring tribal groups.
- Yazdegerd I dies after a 21-year reign and is succeeded by his son Bahram V, who becomes head of the Persian Empire.
- Abdas, bishop of Susa, is accused of burning down one of the fire temples of Ahura Mazda.
- The Jin Dynasty ends in China. Liu Yu (Emperor Wu of Liu Song) becomes the first ruler of the Liu Song Dynasty. Nanjing is reinstated as the capital of northern China.
- The Southern Dynasties begin in China.
- Guisin becomes king of the Korean kingdom of Baekje.
- Anthemius, emperor of the Western Roman Empire (approximate date)
- Ecdicius, Roman general (magister militum) (approximate date)
- Glycerius, emperor of the Western Roman Empire (approximate date)
- Libius Severus, emperor of the Western Roman Empire (approximate date)
- Majorian, emperor of the Western Roman Empire (approximate date)
- Palladius, caesar and son of Petronius Maximus (approximate date)
- Valamir, king of the Ostrogoths (approximate date)
- Yuan Can, high official of the Liu Song Dynasty (d. 477)
- January 21 – Yazdegerd I, king of the Sassanid Empire
- February 26 – Saint Porphyry, bishop of Gaza (Palestine)
- September 30 – Saint Jerome, priest and translator of the Bible
- Saint Abdas, bishop of Susa (Iran)
- Li Xin, duke of the Chinese state Western Liang
- Orosius, Christian historian and theologian (approximate date)
- Pelagius, British monk (approximate date)
- Yao, empress consort and wife of Mingyuan