there are a couple of reasons for not scheduling sunday for shooting Nationals (and i am sure Sean can correct or add to my comments.) First off the Banquet is sunday night. For the last few years we have encouraged people to come to this dressed up. this is hard to do if you are on the range shooting till 5 and have to be at the banquet for 6. it is our one "formal" event of the year as a National group and I think showing up in something better then range attire is appropriate.
Another reason to keep sunday in reserve is in case of a back up during the normal days of shooting. this allows time to finish the match. Back ups could be caused by weather or having to repair a stage. consider it a form of match insurance.