Having the holiday the first Friday of July will create a three day weekend in the middle of the summer between Memorial Day and Labor Day.
When the holiday falls in the middle of the week it does not allow for families to celebrate together, and causes the economy to suffer due to work disruptions. When the 4th is on the weekend it is already moved to a weekday to create a three day weekend.
Having the holiday on a Friday will also allow for fireworks displays to go late without concerns over work the following day.