Historical timeline for May 20

Russo-Ukrainian war: Russia claims full control of the Ukrainian city of Mariupol after a nearly three-month siege.

The International System of Units (SI): The base units are redefined, making the international prototype of the kilogram obsolete.

The government of Singapore authorises the controversial execution of convicted murderer Kho Jabing for the murder of a Chinese construction worker despite international pleas for clemency, notably from Amnesty International and the United Nations.

An EF5 tornado strikes the Oklahoma City suburb of Moore, killing 24 people and injuring 377 others.

At least 27 people are killed and 50 others injured when a 6.0-magnitude earthquake strikes northern Italy.

Mamata Banerjee is sworn in as the Chief Minister of West Bengal, the first woman to hold this post.

An Indonesian Air Force Lockheed L-100 Hercules crashes in Magetan Regency, killing 99.

The independence of East Timor is recognized by Portugal, formally ending 23 years of Indonesian rule and three years of provisional UN administration (Portugal itself is the former colonizer of East Timor until 1976).

Civil rights: The Supreme Court of the United States rules in Romer v. Evans against a law that would have prevented any city, town or county in the state of Colorado from taking any legislative, executive, or judicial action to protect the rights of gays and lesbians.

The first post-Communist presidential and parliamentary elections are held in Romania.

The Chinese authorities declare martial law in the face of pro-democracy demonstrations, setting the scene for the Tiananmen Square massacre.

Radio Martí, part of the Voice of America service, begins broadcasting to Cuba.

Church Street bombing: A car bomb planted by UMkhonto we Sizwe explodes on Church Street in South Africa's capital, Pretoria, killing 19 people and injuring 217 others.

First publications of the discovery of the HIV virus that causes AIDS in the journal Science by a team of French scientists including Françoise Barré-Sinoussi, Jean-Claude Chermann, and Luc Montagnier.

In a referendum in Quebec, the population rejects, with 60% of the vote, a government proposal to move towards independence from Canada.

In the Chuknagar massacre, Pakistani forces massacre thousands, mostly Bengali Hindus.

The Battle of Hamburger Hill in Vietnam ends.

The Popular Movement of the Revolution political party is established in the Democratic Republic of the Congo.

One hundred twenty-one people are killed when Pakistan International Airlines Flight 705 crashes at Cairo International Airport.

Discovery of the cosmic microwave background radiation by Robert Woodrow Wilson and Arno Penzias.