The Watertown-Belmont Chamber of Commerce and town officials are starting what they hope will be an annual tradition with the first official Holiday Tree Lighting Ceremony in Watertown Square on Thursday at 6:30 p.m. Santa will arrive at the festivities by firetruck, and then head over to Town Hall for a performance by carolers from the Perkins School for the Blind. Chamber member Diane Nicolazzo said the business organization is excited about “the start of something new,” especially since surrounding communities all have holiday traditions. “I think it will be big,” she added. Light refreshments, including cookies, hot chocolate, and Munchkins donated by Dunkin’ Donuts, will be served. For more information, call the chamber at 617-926-1017.