September 1 is Disaster Prevention Day in Japan, commemorating The Great Kantō Earthquake (関東大地震 Kantō dai-jishin) of 1923. The quake had a magnitude of 7.9 and devastated Tokyo, the port city of Yokohama, and many surrounding prefectures. Estimated casualties are estimated at 142,800 deaths.
Japanese commentators interpreted the disaster as an act of divine punishment to admonish the Japanese people for their self-centered, immoral, and extravagant lifestyles. The long run response was a strong sense that Japan had been given an opportunity to rebuild the city and its values.
Beginning in 1960, September 1 is designated as Disaster Prevention Day to commemorate the earthquake and remind people of the importance of preparedness, as August and September are the peak of the typhoon season. Schools and public and private organizations host disaster drills.
Summarized from Wikipedia.