which days of July and August 07 are the best time to take GMAT?

NNs,

I'm planning to take GMAT in washington d.c. this summer.  I was wondering if you can suggest which days within July - August 2007 are best to take it so I can fully utilize JJ?  Also please advise when the "Ti Ku" (the pool of exam questions) will be changed during summer?

any advices would be highly appreciated!

Thanks!