Pickett’s Charge (technically Pickett, Pettigrew and Trimble’s Charge) is considered the high water mark of the Confederacy on 3 July 1863. It rained on the 4th as the Union sat on Cemetery Ridge and the Confederates on Seminary Ridge.
Roland’s Time Patrol mission is to take one shot on the 4th of July to prevent the Union from making a fatal mistake and doing the reverse of Picket’s Charge into the Valley of Death.
What he didn’t count on was being stuck among the wounded from the previous days’ combat while waiting to take that shot. You can read Roland’s complete mission in a Quickread here. The complete book: Independence Day (Time Patrol)