Points must be used by that date.
Points must be valid (not expired) for each day of the reservation. When trying to book, the system will only allow you to book what will be available for the days you want.
So for example a trip that goes from May 29th - June 5th can only use these points for May 29th - May 31st.
NOTE: If you have any current use year points in a reservation have MS use all your banked in points first. This should happen automatically. However sometimes with booking/cancelling this could get messed up. You could then bank the unused current UY points.