The 2010 edition of National Parks Week begins on April 17 and lasts until April 25 at the nation's 392 national parks. All entrance fees (up to $25 for a big park like Yellowstone) are waived during the week.
April might seem a little early for the chillier parks out there (Glacier National Park in Montana—which is celebrating a birthday this year—comes to mind). But as we advise in our Beat the Crowds feature, April is a great time to visit California's Death Valley and Joshua Tree parks and Arizona's Saguaro.
Two other days will be fee-free this year: Public Lands Day (Sept. 25) and Veterans Day (Nov. 11).