When is last day of summer 2009 in the us?

It depends on what you're referring to - the dates of the astronomical seasons are the same everywhere on the Earth (except for the difference between the northern and southern hemispheres), because astronomical seasons are based on the position of the Earth in its orbit around the Sun. for example, the last day of summer in Illinois (and the rest of the northern hemisphere) is September 22.

You might also be talking about Labor Day, which is the traditional end of summer for a lot of businesses, pools, etc, because it's the last big summer family holiday. Labor Day is September 7 this year (all over the US).