On August 15, 1281, the Mongolian fleet of Kublai Khan was destroyed by a “divine wind” for the second time in the Battle of Kōan.
On August 15 in 1971, President Richard Nixon ended convertibility of the United States dollar into gold by foreign investors. The dollar has remained fiat money ever since, but — mysteriously! — did not succeed in retaining its previous value.
But then, the dollar under the previous quasi-gold, Bretton-Woods Agreement wasn’t stable either, which is why Nixon felt compelled to close the gold window.