The largest mass hanging in US history was on the day after Christmas in 1862. 38 Dakota were hung in the town square of Mankato Minnesota.
"In Mankato, at ten o'clock on December 26, thirty-eight (one person was reprieved between the date of Lincoln's order and the execution) prisoners wearing white muslin coverings and singing Dakota death songs were led to gallows in a circular scaffold and took the places assigned to them on the platform. Ropes were placed around each of the thirty-eight necks. At the signal of three drumbeats, a single blow from an ax cut the rope that held the platform and the prisoners (except for one whose rope had broke, and who consequently had to be restrung) fell to their deaths. A loud cheer went up from the thousands of spectators gathered to witness the event. The bodies were buried in a mass grave on the edge of town. Soon area doctors, including one named Mayo, arrived to collect cadavers for their medical research."
http://www.law.umkc.edu/faculty/projects/ftrials/dakota/Dak_account.html