Three soaring flagpoles stand on the plaza outside the entrance to Boston’s City Hall. Two of the flagpoles are almost never disturbed — one always flies the American and POW-MIA flags, and the second displays the flag of Massachusetts. But what flies from the third flagpole varies. The city has a longstanding policy of allowing private groups to hold flag-raising ceremonies on the plaza, when they can replace Boston’s flag on the third pole with a flag of their choosing.
Such programs take place all the time. Private groups regularly gather on the plaza to celebrate the culture of another country and fly its flag, frequently accompanied by music or speeches. Other groups celebrate not countries but causes, and have proudly flown their banners — including the rainbow Pride flag, the Juneteenth flag, transgender rights flag, and the flag of the Bunker Hill Association — from the third pole.
Source: The Christian flag and the Boston flagpole - The Boston Globe