The Change of Flags, as it is called, designating the official transfer of East Florida from Spain to the United States under the treaty of cession, took place at St. Augustine July 10, 1821.
Davis, T. Frederick
"Pioneer Florida: Sidelights on Early American St. Augustine,"
Florida Historical Quarterly: Vol. 23
, Article 5.
Available at: https://stars.library.ucf.edu/fhq/vol23/iss2/5