You may be thinking of Stuart’s cavalry engagement East of Gettysburg on July 3rd. It was part of a three-pronged attack planned by Lee on the Union defenses. Longstreet (which included Pickett) was to lead the main attack on Cemetery Ridge, with Ewell attacking Cemetery and Culps Hills. Stuart was to sweep around the Union flank East of Gettysburg.
Stuart engaged Gregg’s and Custer’s cavalry about 3 miles East of town and was routed. It was not a major action of the day, nor of the entire 3 days. His cavalry had only arrived the day before, having been cut off from communication with Lee’s main army for a considerable period, thereby giving Lee no actionable intelligence during his march into Pennsylvania.
I am sure that was it.