At what point in the calender should GMAT be taken?

Well...there is no answer to this! You can take the test at any point you think you are ready with preparation. There are certain myths that the GMAC s/w keeps on getting easy and difficult...even if there is any truth attached to it, you can do nothing about it. Hence, give no heed and take the test when you think that you are at the peak of your preparation...:)