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 can happen at any time before the civil war if slavery is allowed, and any non-slave state has a consciousness of at least six. Furthermore the upper house needs to have at least 10% liberals.
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.
It increases all Dixie POPs militancy by 1 and consciousness by 2, but the most important effect is that it makes the civil war possible.