On December 6, 1917, the SS Mont-Blanc, a French ship laden with ammunition, collided with another vessel in the harbor in Halifax, Nova Scotia. The Mont-Blanc caught fire and its ammunitions exploded, causing devastating damage to the city. Estimates indicate that between 1,700-2,000 people were killed by collapsed buildings, flying debris, and other after-effects of the explosion. More than 9,000 were injured. Some sources claim the explosion was the largest man-made blast before the invention of nuclear weapons. The explosion was so large that it created a tsunami that wiped out all the inhabitants of a First Nations settlement on the other side of the harbor.
Relief came from around Canada and American cities like Chicago, but Boston made particularly generous contributions. When Bostonians learned of the disaster, they organized a relief train that included medical personnel and supplies. The train reached Halifax on December 8 (it was delayed one day due to a snowstorm).
To show their appreciation (and highlight tourism and trade opportunities), Nova Scotia Christmas tree farmers sent Boston a tree for the holidays in 1918. This idea was revived in 1971, and since then the government of Nova Scotia has sent an official Christmas tree to Boston every year. They even have guidelines to ensure the quality of the tree (it must be a white or blue spruce of a certain height and density).
Each year, the city of Boston holds a ceremony to light the tree, and it stands on Boston Common throughout the season.
Read about the 2014 tree.
Read more about the Halifax Explosion.