Emperor Diocletian issues four edicts aimed at destroying Christianity; churches are to be dismantled, clergymen arrested, and their followers forced to sacrifice to pagan gods on pain of death. Those put to death include Agnes of Rome, a 12-year-old Christian girl who has refused marriage and consecrated her virginity to God. Hailed as a martyr, she will be honored as the patron saint of chastity, gardeners, rape victims and virgins.
Constantius Chlorus defeats the Alemanni and fortifies the town of Konstanz (Germany).
Diocletian fights a campaign against the Carpi on the Danube and becomes seriously ill.
A Triumphal Arch is built for Diocletian in Rome.
Sixteen Kingdoms: The Wu Hu uprising establish under Liu Yuan the Han Kingdom.
Sichuan earns its independence from China.
October 25 – Pope Marcellinus dies at Rome after an 8-year reign. The papal throne will remain vacant until 308.
December 25 – Saint Anastasia (martyred)
Bunseo of Baekje, king of Baekje (Korea)
Saint Afra (martyred by fire)
Saint Agape, Chionia, and Irene (martyred)
Saint Agnes (martyred)
Saint Alban (possibly 309)
Saint Florian (martyred)
Saint Gorgonius of Nicomedia (martyred)
Saint Juliana of Nicomedia (martyred)
Saint Lucie of Syracuse (martyred)
Saint Margaret (martyred)
Saint Pancras (martyred)
Saint Philomena (martyred)
Saints Theodora and Didymus (martyred)
Saint Vincent of Saragossa (martyred)
Year 304 (CCCIV) was a leap year starting on Saturday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Valerius and Valerius (or, less frequently, year 1057 Ab urbe condita). The denomination 304 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.