The significance of the April 1865 event that marked the end of the Civil War was the surrender of Confederate General Robert E. Lee to Union General Ulysses S. Grant at Appomattox Court House in Virginia. This event represented the effective conclusion of the Civil War, ultimately leading to the preservation of the Union and the abolition of slavery. The surrender symbolized the defeat of the Confederacy and laid the groundwork for the Reconstruction era, a challenging period aimed at rebuilding the South and integrating formerly enslaved individuals into society as citizens with rights.