If you’re thinking of visiting a U.S. National Park this summer, you’d better plan ahead starting now.

At many parks, your entrance may require a reservation, and not just for a particular day, but for a specific time on that day. The timed reservation requirement will be enforced at Arches National Park and Yosemite.

The reservations rule also will be active at Glacier National Park, Mount Rainier National Park, and Rocky Mountain National Park.

And just because you want to make a reservation at some parks, there’s no guarantee you’ll actually get one at Zion National Park in Utah. It’s a lottery system that controls who gets the reservations.