"Nudity is legal in national parks — little known fact"
Actually, this is a widely spread falsehood, based on the fact that "technically, federally owned U.S. Forest Service and BLM (Bureau of Land Management) land doesn't have any federal law banning public nudity". Yet, "each national forest, grassland, and all BLM lands honor state and county laws" (all this from the same article). Which means that nudity is no more or less legal in national parks than it is anywhere else. Which is to say: barely (if that).
That "BLM officials have said that they will take situations of public nudity on a case-by-case basis" doesn't provide much in the way of comfort for anyone wishing to engage in nude recreation on public land unmolested. That it's not strictly illegal is not the same as saying that it is legal, particularly when it can so easily be misconstrued (by a usually unsympathetic audience) as "indecency". The best advice I could give anyone determined to take the risk is not to get caught, for all that's worth.
As a result, holidays like "Nude Hiking Day" are very difficult to celebrate, even for those fervent adherents to the lifestyle. That's a cause that the nudist community ought to take up, for the benefit of every one of its members. And yet, one need only look at the treatment Stephen Gough (the "Naked Rambler") has received to find out how those who dare to stand up for their essential liberties are treated (sometimes even from within the nudist community, sadly).