Of course, a successful 'henge is dependent on the weather. Last year, a wall of clouds over New Jersey spoiled everyone's view of the solar-street alignment, which is just so typical New Jersey. As of Wednesday night, the National Weather Service's forecast doesn't look very promising -- with the "chance of showers and thunderstorms before 10pm" on Sunday night (half sun) and "Mostly cloudy" skies on Monday night (full sun). But we're still a few days out, so there's a chance the predictions will change.
If the weather does indeed suck on both nights, or if you're too busy grilling and drinking all weekend, your second and last shot at witnessing Manhattanhenge this year will be Monday, July 11th, at 8:20pm (full sun) and Tuesday, July 12th, at 8:20pm (half sun), according to the AMNH. Or you can always continue to ignore the beautiful sunsets like you usually do.