A well-timed family vacation can mean a free visit to a national park this summer. For three weekends, June 20-21, July 18-19 and August 15-16, the National Park Service will waive admission fees for every one of the parks it operates.

Spend a weekend with the kids hiking the red stony yawn of the Grand Canyon or seeing the sunrise at Acadia National Park or puttering around fossil beds in South Dakota’s badlands. For free.

“During these tough economic times, our national parks provide opportunities for affordable vacations for families,” Secretary of the Interior Ken Salazar said in a statement. “I encourage everyone to visit one of our nation’s crown jewels this summer and especially to take advantage of the three free-admission weekends.”

Most Americans live less than a day’s drive from one of the country’s 391 national parks, Salazar said. Last year 275 million tourists visited, generating about $10.6 billion for local economies and supporting more than 213,000 jobs.

More than one-third of the country’s national parks – 147 to be exact – charge an admission fee, ranging between $3 and $25.

The summertime waiver does not include other fees collected by the parks, such as charges for camping or making a reservation. However, many businesses that rely on the parks, such as hotels, restaurants and tour operators, will offer discounts those same weekends, making a family vacation even more affordable.