The explosion in Halifax, Nova Scotia in December 1917 was caused by a munitions ship leaving for Europe to supply the British during World War I. About 2,000 people were killed and the harbor was destroyed; the anchor of the ship was found 2 and half miles inland, where I'm told it's been left where it landed as a memorial.
Boston was the first city to respond with aid, being just a day's sail from Halifax, and at the time, the preeminent medical authority in the US, as well as one of the busiest ports. So off went shiploads of supplies, disaster relief, and medical personnel.
The people of Halifax were so grateful for the aid that a few years later, after they had gotten back up on their feet, they sent a 100 foot pine tree to Boston at Christmastime as a thank you.
...and they've been doing it every year since.
The True Meaning of Christmas? All I know is that there's a nearly century-old thank-you tradition between the peoples of Boston and Halifax, and I get to enjoy their Christmas tree every year.