Boston's official Christmas tree has arrived at Boston Common.The tree was harvested in Nova Scotia last week and participated in two parades prior to starting the trip of over 680 miles to Boston. The annual gift is a show of gratitude for Boston's help after the Halifax Explosion in 1917.On Dec. 6, 1917, two ships collided in the Halifax Harbour, causing a devastating explosion that killed nearly 2,000 people and left hundreds more severely injured and homeless. Boston then dispatched a train full of supplies and emergency personnel within a day of the disaster.This is the 48th year that Nova Scotia has shown its thanks to the city of Boston by gifting a Christmas tree. The 2019 tree is a 60-year-old, 45-foot white spruce, donated by Desmond Waithe and Corina Saunders of Black Point, Pictou County.Local children and an official Nova Scotian town crier are typically among the crowd greeting the tree as it arrives on Boston Common. It will then be installed in a position of honor and strung with lights for the city's official tree lighting ceremony Dec. 5.WCVB will broadcast the Holiday Lights show live from Boston Common starting at 7 p.m.