EVERY CHRISTMAS EVE, hundreds of people from all over New England brave the cold and the lack of parking to pack the streets around Beacon Hill’s Louisburg Square for a ritual tintinnabulation. At 8 p.m., a single silvery note cuts through the chatter, its bright chime reverberating in the dark. Voices rise, and the spectators become a chorus crooning familiar verses of holiday carols: “God Rest Ye Merry Gentlemen,” “Joy to the World,” “Silent Night.” People hold hands, embrace friends, smile at strangers. In the air, there’s a feeling of Christmas.