Weekday or weekend? As others are saying, parking outside and taking a train in is probably universally your best bet. If there are other considerations that make that a disfavored option, I would highly advise against trying to park in the North End. You won't find any street parking in the part of the city that you're looking at that has 6+ hour parking, so a garage would really be your only option. Metered parking IS free after 8 and all day on Sunday, so you could technically do it from 6:01pm-8pm (it's usually max two hours) and then be fine if you stay longer. If you're coming on a weekend, I've never had any trouble finding parking in the Financial District. That said, there are some very cheap (like $10 all-day) garages that I would still recommend for ease of having an actual address to navigate to.