The Battle of Gettysburg was fought from July 1 to July 3, 1863. Robert E. Lee took the Army of Northern Virginia into Pennsylvania with the hope of drawing the Union Army of the Potomac, under the command of General George Meade, away from Virginia.  Lee’s army moved without the guidance of its cavalry under the leadership of General J.E.B. Stuart. Unaware of a Union presence, the army was then drawn into battle. The two armies clashed in Gettysburg for three days. The battle resulted in over 53,000 casualties combined.  Lee’s army was forced to retreat into Virginia because it failed to destroy Meade’s army. The battle is regarded as the turning point of the Civil War in the East. It was also the bloodiest battle of the war. 
For a more detailed look at the events of the battle, this video summarizes what took place.
 Whitelaw Reid, Arthur James Lyon Fremantle, and Gary W. Gallagher, Two Witnesses at Gettysburg: The Personal Accounts of Whitelaw Reid and A.J.L. Fremantle, 2nd ed. (Malden, MA: Wiley-Blackwell, 2009), xiii.