Recreation.gov covers federal land: national parks, national forests, Army Corps sites and BLM. ReserveCalifornia covers California's state parks and state beaches. Plenty of California destinations sit in one system or the other, and knowing which decides when you should be watching.
Release times differ. Recreation.gov opens inventory at 4:00 PM Eastern, ReserveCalifornia at 8:00 AM Pacific. Both run booking windows of roughly six months. Because the two are unrelated systems, a trip combining a national forest campground with a state beach is really two separate booking exercises on two different clocks.
The data differs too. ReserveCalifornia exposes site-level detail — site type, vehicle length, loop — which is why Kestrel's Today's Releases page can show individual campsites grouped by park and loop. Recreation.gov's availability endpoint returns site identifiers and dates but less descriptive metadata, so its campground pages lean on the 14-day availability calendar instead.
Both systems are the authority on their own inventory. Kestrel reports what its most recent scan observed and links straight to the official booking page; availability can change between a scan and your click, so the provider's page is always the final word.