For those following the Civil War day-by-day, the Rosh HaShanah dates during the war
1861 (5622): Sept. 5
1862 (5623) Sept. 25
1863 (5624) Sept. 14
1864: (5625)Oct. 1
For Yom Kippur add 10 days. Kind of looks like the armies managed to avoid major actions on the Jewish high holidays - funny coincidence.