You cannot modify the dates, unit size, or view category of an automatically-booked fixed week in any way. If you only want five of the seven days you either cancel and rebook, or check out early and use the extra points. Here is the relevant passage from the Home Resort Rules:
The exception about Special Event Rights, as far as I know, only apply to the runDisney weeks that are no longer being sold. Because those races typically include a Sunday, you would (a) automatically be booked the Sunday-to-Sunday ending on race weekend, and (b) can drop beginning days to book days after the end once the 11-month window opens, similar to how walking happens.
I suspect that the potential for shifted race weeks to partially overlap with non-race weeks is one reason why they don't sell the race weeks anymore.