An annual gift of gratitude between a pair of East Coast cities has taken on a special meaning this year.

Every year, the city of Halifax donates a Christmas tree to the city of Boston as a gift for their help after the 1917 Halifax explosion.

December 6 will mark 100 years since the tragedy, where two ships collided in the harbour -- one packed with explosives.

The explosion is known as the world’s largest man-made blast before the nuclear bomb. It took the lives of nearly 2,000 Haligonians, while leaving another 9,000 injured and 25,000 homeless.

Following the blast, the city of Boston sent nurses, doctors, supplies and funds to Halifax.

"I am proud of our city stepping up in such a big way,” said Boston Mayor Marty Walsh. “I admire the people of Halifax for their resilience."

On Thursday, Boston unveiled the tree to the public with a lighting ceremony at the Boston Common.

For Halifax, the tree has become a promotional tool for tourism, but also serves as a genuine symbol of thanks for the help Bostonians gave the city a century ago.

"You meet people from Boston, (and) they’re touched by the story and they do feel a connection to it,” said Halifax Mayor Mike Savage told CTV News. 

Providing the gift has become a source of pride for many Nova Scotians. There is even an application process for trees to be considered.

This year’s tree is from Blues Mills, a small community in central Cape Breton.

For some Nova Scotians, witnessing the tree fully lit up is something they can’t miss.

“It's been nice to be able to follow the tree down and take part in it this year," said Linda Pickles, who drove from Halifax to see the ceremony.

In addition to the tree, the Province of Nova Scotia unveiled a plaque in Boston this year to mark 100 years since the explosion.

With a report from CTV’s Atlantic Bureau Chief Todd Battis in Boston