The Christmas tree is up in London's main square, which means it's officially Christmas :-)
Each year since 1947, a Christmas tree has been given to the people of London from the people of Norway in gratitude for Britain's support for Norway during World War II. The tree is usually a Norwegian spruce (Picea abies) 21 metres (70ft) high and 50-60 years old. It is selected from the forests surrounding Oslo several months in advance.
The tree is felled in November during a ceremony in which the Lord Mayor of Westminster, the British ambassador to Norway and the Mayor of Oslo participate. It is brought to the UK by sea, then completes its journey by lorry. A specialist rigging team erects it in Trafalgar Square in the centre of London, using a hydraulic crane. It is decorated in traditional Norwegian fashion, with vertical strings of lights, using energy-efficient bulbs.
The lights were lit yesterday by the Mayor of Oslo and the Lord Mayor of Westminster. The ceremony began at 6 PM with carols sung by the Choir of St Martin-in-the-Fields, accompanied by the Westminster Salvation Army band. Following gale force winds last night, this morning the tree looked
a bit wonky, but it has since been propped back up.

The Christmas tree provides a central focus for traditional carol-singing. Last year over fifty different groups of carol singers from across the country took to the stage, and this year many different choirs will take part, singing to raise money for good causes.
The tree will stay in Trafalgar Square until 4 January, just before the twelfth night of Christmas, when it will be taken down for recycling. The tree is chipped and composted, to make mulch.
For more information, see
here.