LONDON (AP) — London's Christmas tree in Trafalgar Square will be lit up Thursday as part of a British tradition with roots in the Norwegian forest.

The tree symbolizes the friendship between London and Oslo in an annual ceremony that highlights the history of the countries' alliance.

Here's how the tradition began:

The bond between the two countries was forged during World War II. When Nazi Germany invaded Norway in 1940, King Haakon VII and his government-in-exile fled to London. Much of Norway's resistance work grew out of Britain.

The BBC, and its Norwegian counterpart NRK, broadcast the news in Norwegian from London. The reports were a vital lifeline for Norwegians, who listened in secret because of the Nazi radio ban.

As a token of its thanks, the Oslo city government established the Christmas tree gift in 1947, two years after the war ended.

The tree is always a Norwegian spruce, or Picea abies, grown naturally in the Oslo forest. A few are usually selected as possibilities five to 10 years ahead of time and receive extra care leading up to the final selection.

Nicknamed “Ever Oslo,” this year’s tree is some 20 meters (22 yards) tall and around 60 years old. It beat out two other trees, dubbed “Nordic Star” and “Fjord Fir,” for the honor. The felling ceremony occurred Nov. 21.

After the tree was ceremoniously cut Nov. 21, it was loaded into a special cradle and driven about 180 kilometers (112 miles) to a port, where it was rinsed to remove salt accumulated from the roads.

From there, it traveled roughly 26 hours aboard a ship — stored below deck to protect it from saltwater — to Immingham, about 239 kilometers (149 miles) north of Trafalgar Square in London. It continued on by truck to the British capital.

A replacement tree will be planted in the forest outside Oslo.

Billed on social media as “Britain’s national tree-sure,” the lighting ceremony signals the start of London’s countdown to Christmas and is traditionally held on the first Thursday in December.

Strings of lights hang vertically, following the Norwegian tradition.

The tree will remain in the square until Jan. 5, also known as the Twelfth Night of Christmas. It will then be taken down to be chipped and composted for mulch.