Out-of-state visitors driving in need an entry reservation with parking, released on the same rolling 30-day window.
Hawaiʻi State Parks' own Diamond Head page says out-of-state visitors need an advance reservation, bookable up to 30 days ahead, that busy times may sell out, and that "Persons without a reservation will be turned away." Hawaiʻi residents are exempt. Refunds are allowed up to three days before the entry date, which returns slots to the pool. We watch for open slots and alert members.
Updated , checked by hand against the target's own page.
Watched from gostateparks.hawaii.gov, the target's own public page — not a reservation system we poll.