Are there any available campsites in the park right now?
Campgrounds are usually full on weekends October through May. From mid-February to mid-May (and during holidays) campgrounds usually fill throughout the week. To improve your chances of getting a campsite and avoid disappointment, visit during the off-season (June-September), reserve a site, and/or have alternate overnight plans ready.
Since most park campgrounds are first-come, first-serve availability can change by the minute.