On only four different days of 2018, all National Parks charge no entrance fee. One is Martin Luther King, Jr. Day. (The others three are: National Park Week Apr 21, National Public Lands Day Sept 22 and Veterans Day Nov 11.)

According to the National Parks Service free days "provide a great opportunity to visit … one of the national parks that normally charge an entrance fee." (Free, of course, does not include merchandise, certain activities, tours, etc …)

Wyoming’s National Parks that normally do charge entry fees are:

Bighorn Canyon National Recreation Area

Devils Tower National Monument

Grand Teton National Park

Yellowstone National Park