This nation-wide event is one of five free entry days celebrated by the National Park Service and those who visit them.
From the event website:
"Look Who's Turning 103! Celebrate the National Park Service's 103rd birthday on August 25. Check the calendar below to find special birthday events at parks. In the meantime, you're invited to continue seeking new experiences and discoveries waiting for you in parks across the county. The National Park Service was created on August 25, 1916, when President Woodrow Wilson signed the National Park Service Act. Each year, we work with partners to celebrate this anniversary. Find a park near you or events happening at parks across the country."
More details on the NPS website here!