Because that's when the Patriots Duck Boat parade will start at the Prudential Center. It will end with at City Hall, rather than with the ceremonial dunking in the Charles, because the Duck Boats are not equipped as icebreakers.
UPDATE: Mayor Walsh says no rally at City Hall, though, due to all the snow and the windchill. Also: City is working to remove snow from everywhere, not just the parade route, mayor says. He adds:
I think we're gonna have school tomorrow. We're not cancelling school for the parade tomorrow, I can tell you that.