The Fourth of July weekend is likely to produce -- by far -- the biggest box-office grosses of the year. Summit Entertainment launches the third installment of its Twilight franchise at midnight tonight (Wednesday/Thursday), amid expectations that it will sell $150-million worth of tickets by Monday. Also opening is Love Ranch, starring Helen Mirren and directed by her husband, Taylor Hackford. It's opening in 11 theaters and not expected to draw big crowds. On Thursday, Paramount premieres M. Night Shyamalan's 3D adventure The Last Airbender , which will be grabbing more than 2,000 3D screens. Analysts are predicting that it will earn $50-60 million. Finally, Disney/Pixar's Toy Story 3 is expected to remain a potent force among family fare and gross around $35-40 million in its third weekend.