1879
Specie resumption
On 1 January, United States Notes are again treated as gold at par in practice.

The Specie Payment Resumption Act of 1875 set the date: 1 January 1879. On that day the United States resumed specie payments. The wartime greenback premium had closed in practice.
Defining the dollar as a gold unit in statute waited until the Gold Standard Act of 1900. Resumption was the operational hinge. The 1900 Act was the label.