ask for help about gmat day

i want to have the gmat test in september in usa,but  i do not know when will the "database" change in the month,and which day is a good day to take the test(the end or the middle)?

hope kindy person give me some suggest!

http://forum.chasedream.com/dispbbs.asp?BoardID=34&ID=114939&replyID=&skin=0

可以用题库作为关键字在困境区搜索一下