On this date:
- Constantine I was proclaimed Emperor by his troops after the death of Constantius Chlorus, and became the first Christian leader of Rome (306).
- Spanish conquistador Sebastián de Belalcázar founded Santiago de Cali in present-day western Colombia while on his search for the mythical El Dorado (1536).
- Charles William Ferdinand, the Duke of Brunswick, issued the Brunswick Manifesto to the population of Paris, promising vengeance if King Louis XVI and other members of the French Royal Family were harmed (1792).
- An atomic bomb is detonated underwater in the lagoon of Bikini atoll; meanwhile, Dean Martin and Jerry Lewis stage their first show together in Atlantic City (1946).
- The Italian ocean liner SS Andrea Doria collides with the MS Stockholm in heavy fog 45 miles south of Nantucket, and sinks the next day, killing 51 (1956).
- Porn star Tera Patrick is born (1976).
- Two pro-independence activists were killed by police at Cerro Maravilla in Villalba, Puerto Rico, sparking a series of political controversies where the police officers were eventually convicted of murder and several high-ranking local government officials were accused of covering up the incident (1978).
- Air France Concorde Flight 4590 from Charles de Gaulle in Paris to JFK in New York crashed in Gonesse, France, killing 113 - 109 on the aircraft, and 4 on the ground (2000).
It's also my birthday, and the "official" start of my birth year, when my age and the last two digits of my year of birth are the same number, a once-in-a-lifetime occurrence.