The National Park Service will allow visitors free entry during three weekends this summer to encourage Americans to visit national parks such as the Grand Canyon, Yosemite and Rocky Mountain National Park. The fee waivers -- one each month in June, July and August -- will apply to all 391 national parks across the country, from Alaska to Florida. Interior Secretary Ken Salazar says the free weekends should help American families struggling with the recession afford a memorable vacation. All 147 Park Service sites that charge fees for entry will waive entrance fees from June 20-21, July 18-19 and August 15-16. Entry fees being waived range from $3 to $25.