Latest date you can reserve is the latest date you can find an open spot in the loop, or in the park sayruh1. As long as you can find a spot to set up the pup you're more than welcome to come on down! Do it quickly though, bear in mind this will be during spring break and sites are going fast!
Hope you can make it sayruh1! According to their website, reservations can be made until 4 days prior to arrival. Call National Recreation and Reservation Service at 1-877-444-6777 or reserve online at www.recreation.gov.
If you schedule and have to cancel, there is a $10 cancellation fee that will be deducted from your refund. (Unless you don't notify them and are just a no-show.)
At the suggestion of ryanincc I would like for everyone's yay or nay on renaming this rally. It would become an annual tradition, always in march, this upcoming rally would be the first of an annual "Nipress Spring Rally"?