John Brown's Last Raid
John Brown's Last Raid is a major event for USA and an integral part of The American Civil War. It is one of the two events, that needs to fire before the event starting the civil war, A House Divided (event), can fire. The other is Dred Scott v. Sandford. The requirements for the two events are very similar, but one is for slave state, and the other is for not-slave states.
It has a mean time to happen of 200 months before 1845, of 100 months before 1855 and 50 months before 1860. After 1860 it has a mean time to happen of only 10 months.