“Buffalo Soldiers Day on July 28 commemorates the formation of the first regular Army regiments comprised of African American soldiers on September 21, 1866, seventeen months after the end of the Civil War. Established by Congress, these regiments were the first peacetime, all-black regiments in the regular U.S. Army.”
https://nationalvmm.org/buffalo-soldiers-day-july-28/
Learn more:
https://www.nps.gov/subjects/buffalosoldiers/index.htm