You are not being measured relative to other test takers on a particular day but rather the curve is already set when you take it, based off of previous years' data. Now the exact formula they use to control for discrepancies among testing population on a given date is not known to the public but you can surmise that the any "advantage" you get from taking it on any particular date is negligible. You should take it when you are the most prepared for it which for most people in school is in the summer months after dedicated study time but in your case, you can take it whenever.