I agree with Rick. The same dates every year is the best thing to do.
We already do this to a certain extent when bidding on a race. I look at last years date, see if it works for us and try to keep it at the same time.
Having nationals on the same date would make planning regional races easier as well. This way a regional isn't scheduled before a national on the same date. I know we plan our two big carpet races well in advance around birds, cleveland and the roar carpet nats.