It was also a Wednesday night. A smaller crowd shows up then. He probably had visited there before.
On the South Carlina Emergency Management Division Training and Events Calender, for the date of June 17, 2015(the date of the shooting) we read
docs.google.com/viewer?url=www.scemd.org/files/Training/24monthcalendar.pdf
“SC National Guard ROC(Rehearsal of Concept) drill (Charleston)”
and
“Southern Exposure Dress Rehearsal”
No further info on what these things are, or if they had some connection with the church massacre.