If you want to visit a National Park, and don't mind crowds, Saturday is the day for you.
Via the NPS:
There are many ways to participate in National Public Lands Day.
You can visit a national park for free.
You can take part in a volunteer work project.If you volunteer on this day, you will receive a fee-free day coupon to be used on a future date.
You can share your favorite outdoor activity on social media channel with the hashtag #NPSVolunteer, #FindYourPark and #NPLD!