Obviously not all of these parks are camping parks, but any day outside is a good day.
Cut and pasted from the news site ...
"National park entrance fees will be waived 17 of the following days next year.
January 14-16 (Martin Luther King Jr. weekend); April 21-29 (National Park Week); June 9 (Get Outdoors Day); Sept. 29 (National Public Lands Day) and Nov. 10-12 (Veterans Day weekend).
There are still a few free days left for 2011 as well. All of the national parks will once again allow entry to travelers without paying on Veterans Day weekend, which is Nov. 10-12."
Here is the whole article ...
Link for the specific parks ...
Hope this helps someone get some fresh air.