1972 followed by 1973
1972 (MCMLXXII) was a leap year starting on Saturday of the Gregorian calendar, the 1972nd year of the Common Era (CE) and Anno Domini (AD) designations, the 972nd  year of the 2nd millennium, the 72nd  year of the 20th century, and the  3rd   year of the 1970s decade.  
Within the context of Coordinated Universal Time (UTC) it was the longest year ever, as two leap seconds were added during this 366-day year, an event which has not since been repeated. (If its start and end are defined using mean solar time [the legal time scale], its duration was 31622401.141 seconds of Terrestrial Time (or Ephemeris Time), which is slightly shorter than 1908).December
December 2 – 1972 Australian federal election: The Labor Party led by Gough Whitlam defeats the Liberal/Country Coalition government led by Prime Minister William McMahon. Consequently, Whitlam becomes the first Labor Prime Minister of Australia since the defeat of Ben Chifley in 1949. Whitlam will be sworn in on December 5th; his first action using executive power is to withdraw all Australian personnel from the Vietnam War. McMahon resigns from the Liberal leadership almost immediately; he will be replaced by outgoing Treasurer Billy Snedden.
December 7
Apollo 17 (Gene Cernan, Ronald Evans, Harrison Schmitt), the last crewed Moon mission to date, is launched and The Blue Marble photograph of the Earth is taken. The mission also includes five mice.
Imelda Marcos is stabbed and seriously wounded by an assailant; her bodyguards shoot the assailant.
December 8
United Airlines Flight 553 crashes short of the runway, killing 43 of 61 passengers and two people on the ground. A day later, over $10,000 cash is found in the purse of Watergate conspirator Howard Hunt's wife, who was on board.
International Human Rights Day is proclaimed by the United Nations.
December 11 – Apollo 17 lands on the Moon.
December 14 – Apollo program: Eugene Cernan is the last person to walk on the Moon, after he and Harrison Schmitt complete the third and final Extra-vehicular activity (EVA) of Apollo 17. Planning for the next crewed mission to the Moon does not begin until 2017.
December 15
The Commonwealth of Australia ordains equal pay for women.
The United Nations Environment Programme is established as a specialized agency of the United Nations.
December 16
The Constitution of Bangladesh comes into effect.
The Portuguese army kills 400 Africans in Tete, Mozambique.
December 19 – Apollo program: Apollo 17 returns to Earth, concluding the program of lunar exploration.
December 21 – ZANLA troopers attack Altera Farm in north-east Rhodesia.
December 22
Australia establishes diplomatic relations with China and East Germany.
A peace delegation that includes singer-activist Joan Baez and human rights attorney Telford Taylor visit Hanoi to deliver Christmas mail to American prisoners of war (they will be caught in the Christmas bombing of North Vietnam).
December 23
The 6.2 Mw Nicaragua earthquake kills 5,000–11,000 people in the capital Managua. President Anastasio Somoza Debayle is later accused of not distributing millions of dollars worth of foreign aid.
Swedish Prime minister Olof Palme compares the American bombings of North Vietnam to Nazi massacres. The U.S. breaks diplomatic contact with Sweden.
Braathens SAFE Flight 239, also known as the Asker Accident: An aircraft crashes during approach to Oslo Airport, Fornebu, Norway; forty people on board are killed.
December 28 – The bones of Martin Bormann are discovered in Berlin.
December 29 – Eastern Air Lines Flight 401 crashes into the Everglades in Florida, killing 101 of 176 on board. It is the first hull-loss of a wide-body aircraft.
December 31 – For the first and last time, a 2nd leap second is added (23:59:60) to a year, making 1972 366 days and two seconds long, the longest year ever within the context of UTC.