The roughly one third of summer campsites never offered for reservation, issued in person at a Wilderness Information Center the day of or the day before a trip.
Mount Rainier's own wilderness permit page says about one third of campsites are not reservable and are issued first come, first served at a Wilderness Information Center, no sooner than one day before the trip and never by phone. Reservations not activated by 12:00 pm on the start day are cancelled and their camps join this walk-up pool. As of this check the page also lists active fires in the northeast of the park, with the Sunrise-area trails, the Northern Loop Trail and part of the Wonderland Trail closed until further notice.
Updated , checked by hand against the target's own page.
No upcoming date is published yet. A watch here alerts you only once a date is recorded on this page.
More openings to watch in Washington
Watched from nps.gov, the target's own public page — not a reservation system we poll.