The St Anthony's Church of Somerville is holding their Festa de São João on the 24 of June. The church is located at 480 Somerville Ave after market basket and near Dane St, I believe it usually starts at 6pm, and the number for the church is 617-625-4530. Although religious festivals (St Peter, St John, St Anthony), the "festas juninas" are also very cultural and more or less oriented toward harvest celebrations, thanking the saints for the rain. This is where the chow comes in, these feasts often celebrate foods made with corn -- grilled corn, pamonha (sort of a sweet tamale), canjica (sweet hominy often with coconut), cake made with cornmeal, etc. Peanuts are also common. Traditional dances and theatre with people dressed up in ragged clothes. They sometimes serve their version of the "hot toddy" "quentão" and I think I have seen that in somerville, although beer is more common. There also is kebobs, etc. These festivals are so popular in parts of the "interior" of Brazil that now they run into July.
One thing I will note is it gets **really** crowded and sometimes hard to move. Would be hard to arrange to meet up, unless it were at the bleachers at the back of festival (sometimes entering from the back is easier).
Allston and Everett also have their own festivals. Allston is usually first and Everett afterwards. When I find out the dates of those I will try to post. There also are a number in metrowest, but I have no idea when. You could check with Brazilian Catholic churches in Ashland, Framingham, and Milford. Brockton or Rockland might also have something similar.