Dred Scott v. Sandford is a major American event 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 John Brown's Last Raid. The requirements and effects of the two events are very similar, but one is for non-slave states and one is for slave states.
It can happen at any time before the civil war if slavery is allowed, and any slave state has a consciousness of at least 5. 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 Yankee POPs militancy by 1 and consciousness by 2, but the most important effect is that it makes the civil war possible together with John Brown's Last Raid.