Every year, Nova Scotia sends a "Tree for Boston" across the Canada-U.S. border and the reason why is going viral.

This weekend via Twitter, the Canadian Armed Forces in the U.S. shared the over one-hundred-year-old story of why a Christmas tree is sent from Canada every year, and displayed in Boston Common.

This tradition is to honour the help received when two ships collided in Halifax Harbour, way back in December 1917, which resulted in an explosion and a tsunami.

Over 1,500 people died instantly, and another 400 people lost their lives in the following days.

Many others were injured as a result.

Editor's Choice: You Can Get $400 Back From The Government If You've Worked From Home During The Pandemic

To say we remember. To say we'll never forget. To say thank you, neighbors. Canadian Armed Forces in the U.S.

Just one day later, a blizzard hit and 25,000 people were without homes.

"When word reached Boston, details were scant. Governor Samuel McCall offered assistance immediately via telegraph, but dispatched a train before receiving a response," the CAF explained.

The equivalent of $1.9 million was raised within an hour by a relief committee in the city, and a train left Boston less than 12 hours after the explosion in Halifax Harbour.

Doctors, nurses and supplies were onboard.

According to the CAF, Americans helped with relief efforts, built temporary housing, ordered supplies, treated patients and stayed there for months afterward.

So, the Christmas Tree is sent each year to remember the help and to say thank you!

This year, the story went viral on Twitter, as the CAF's tweets received over 12,000 retweets and 41,000 likes.