Just remember that doing this, PBS will prioritize the day chronologically. For example, prefer-off Saturday will try to award the 1st Sat of the month first, then the 2nd, etc...

To the OP, if you're 17 in category, just request off the days you want, you'll like get them. As a backup, you could add prefer off weekends...heed the advice of putting a min of X if you want more than 1 weekend off. If you just put prefer off weekends, if PBS awards you just one weekend off, it will be honored.