For a single day, I think there are a few days of the Indonesian anti-communist purges that would qualify. Overall, about ½-1 million were killed, over a period

For a single day, I think there are a few days of the Indonesian anti-communist purges that would qualify. Overall, about ½-1 million were killed, over a period of a few months, but a lot of the deaths happened over a period of just a few days when the army rolled up into central and eastern Java.