On August 27, 1928, the Kellogg-Briand Pact was signed in Paris by 15 nations, including the United States, and many other nations signed on later. Originally conceived as a treaty between France and the U.S., it was soon broadened to include other nations. The Pact's formal name, General Treaty for the Renunciation of War, indicates the purpose of the treaty -- for nations to renounce war as a primary means of settling disputes. It was ratified by the Senate and appears to be a matter of U.S. law that's still on the books.
Suggestions for celebrating the idea of peace on this anniversary include a Let there be peace on earth sing-along or John and Yoko's Give peace a chance.