I think you'll need to ask your school for their schedule, as the window probably takes into consideration your school start date, breaks, and other commitments. Based on our fall test here being in October, I would think January over December (like maybe Oct-Jan/Feb-April/May?), but then again, we take a nationally normed timed standardized test in January, so I don't know if they'll do both close together.
As for preparation, all I did was remind that it was not timed, and suggested careful reading and focusing on not hurrying to avoid accidental errors (in order for more challenging questions to be reached).
