According to the Romania Insider:
…on May 10, 1866, when Prince Karl of Hohenzollern-Sigmaringen (Carol I) arrived in Bucharest and was proclaimed Ruling Prince of Romania.
…in 1877 during the Russo-Turkish War…again on May 10, Carol I signed the Romania’s proclamation of independence from the Ottoman Empire. Prince Carol then assumed command of the Russo/Romanian army…and brought Romania full independence from the Ottoman Empire.
On May 10, 1881, Carol I was crowned king of Romania. The crown used for his coronation ceremony was made from the steel of a cannon the Romanian Army had captured from the Ottomans during the War of Independence.
To read the entire article in the Romania Insider, please click here.