Visitors to America's national parks will have fewer chances to avoid entrance fees next year, the National Park Service announced last week.

The federal agency says it's cutting its number of fee-free days from 10 in 2017 to just four in 2018. Those days will be Martin Luther King Jr. Day (January 15), the first day of National Park Week (April 21), National Public Lands Day (September 22) and Veterans Day (November 11).

In 2017, the fee-free days included the weekends surrounding National Park Week and all of Veterans Day weekend. This year's fee-free days were six fewer than in 2016, when all of National Park Week and four days around the Park Service's centennial were included.

The reduction drew criticism from conservation advocacy group Western Priorities, which accused Interior Secretary Ryan Zinke of "being determined to turn [national parks] into a playground for the rich." The Park Service, though, says the reduction will increase revenue and allow it to improve facilities and address deferred park maintenance.

For more information about the fee-free days, click here.

The move comes amid a public-comment period on the agency's proposal to increase entrance fees at 17 of its most popular parks, including Grand Canyon National Park in Arizona. Public comments on that plan are being accepted through this Friday, December 22.