4th of July is Independence Day for the Americans, their national day. And to celebrate this day, they really do go full out. They have parades, festivals, music, food, etc., all over the city. But the highlight is probably to go down to the Charles River bank (Charles River runs through Boston and is very long), either on the city center or Cambridge side, to join the hundreds of people for barbeques and to spend time getting to know others. In the evening, they bring out the fireworks and the sky is filled with multicolored lights all over the city. It's beautiful and a very nice experience for everyone of all ages.
And then, of course, the younger people go home and get ready for a night full of dancing down in the Alley, Landsdowne Street, or Faneuil Hall. They almost always have some concerts on in Boston Common as well, a big park in the heart of the city, with famous acts to entertain. Enjoy!