Why did STS 121 launch before 116?

[FAAmecanic]

Just noticed the STS 121 actually DID lift off on 07/04/06  and STS 116 was 12/09/06...  looked it up on NASA website and that is true.

So does anyone know why NASA had STS 121 lifting off prior to STS 116?   Was it a maintenance thing where some of the Shuttles were being maintained/modified and lifted off at a later date maybe?

[davidrobinsonjr]

After the Columbia accident, NASA needed two flights to test all the return to flight items. They needed to sandwhich an extra flight between STS-114 and 115 which were allready scheduled  and the picked the next available flight number: STS-121.

[FAAmecanic]

Ahhh...had a feeling it was a scheduling thing.  Just lost sight of why there was that gap  (shame on me...    )

Thanks.