August 16th, 1946. In the aftermath of WWII Britain is in the process of handing over control of India to the native population. There are two major factions within the Indian government, however, The Muslim League and the Indian National Congress (which was mostly Hindu). The short version is that the Muslim League led by Muhammad Ali Jinnah wanted to form an independent nation, where the population was already predominantly muslim, called Pakistan, and let the other Indian principalities declare for India, Pakistan, or their own independent nations. The Indian National Congress rejected this plan, preferring instead to keep India geographially and politically (i.e. predominatly Hindu) as it was.
The Muslim League reacted to the outright rejection by declaring Direct Action Day, and calling for muslim-operated shops to be closed, people to march in protest, and possibly other things that would be explained to their adherents at the rallies. The exact numbers that turned out are difficult to calculate now because estimates varied so widely depending on the person reporting, but it was somewhere between 30,000 and 500,000; a huge difference.
Tensions had been building for years between the two entities in a city that was at the time 64% Hindu and 33% Muslim. It didn't take long for violence to errupt, and both sides seem to have come prepared and heavily armed. Within 72 hours about 4,000 people are estimated to have died as gangs of men from both sides caught and killed individuals from the other, ransacked shops and homes, creating terror and devestation.
The remaining British Raj seemed determined to stay well out of it, and declared a public holiday which they explained was a way to keep people in their homes where they might have a greater chance at safety, rather that sending them to work where they might be in danger. The violence spread in subsequent days to neighboring towns and provinces, paving the way for the Partition of India to come.