On December 6, 1917, two tramp steamers, the Mont-Blanc and the Imo, collided in wartime Halifax Harbour. Steel grating on steel caused sparks to fly creating what became the largest man-made explosion of its time.

More than 2000 people died (500 of those children) and 9,000 were injured. It left 6,000 people homeless and an additional 19,000 without adequate shelter. The blast destroyed or damaged 6630 buildings.

One of the most generous responses came from Boston and the state of Massachusetts. Within 48 hours it organized trains to carry 33 doctors, 79 nurses, and six representatives of relief agencies. In honour of this act of compassion and generosity, every year the province of Nova Scotia sends the City of Boston one of its most beautiful trees. The tree lighting usually takes place in November on the Boston Common. Traditionally, the Nova Scotia tree is always the first tree lit on the Common.