Space Shuttle Endeavour At International Space Station

"The target dates were adjusted because critical payload hardware for STS-133 will not be ready in time to support the previously planned September 16 launch," NASA said in a statement. With the Discovery launch moving to November, Endeavour mission "cannot fly as planned, so the next available launch window is in February 2011," NASA said, adding that the launch dates are subject to change.
The launch was once again postponed until April 29, 2011, in order to avoid a scheduling conflict with a Russian supply vehicle heading for the International Space Station.
The launch was finally carried out on May 16, 2011. Shuttle Endeavour's last mission, STS-134, was a successful launch at 8:56 EST.
One final mission, STS-135, has since been added to the schedule, and Atlantis is allocated to fly in July 2011.