December 12 is the 346th day of the year (347th in leap years) in the Gregorian calendar.
1787 – Pennsylvania becomes the second state to ratify the United States Constitution five days after Delaware became the first.
1901 – Guglielmo Marconi receives the first transatlantic radio signal (the letter "S" [***] in Morse Code), at Signal Hill in St John's, Newfoundland.
1911 – Delhi replaces Calcutta as the capital of India.
1917 – In Nebraska, Father Edward J. Flanagan founds Boys Town as a farm village for wayward boys.
1950 – Paula Ackerman, the first woman appointed to perform rabbinical functions in the United States, leads the congregation in her first services.