if you camp inland, you might be able to get a site without reservations.
but if it's in the summertime or a coastal park anytime, you can forget about just showing up and expecting that you can get a campsite.
that's why we only camp during the week or the spring and fall. no summer, weekend or holiday camping for us. one is because you have to make reservations 6 months in advance to even get one and two because of all the crowds during those times.
we're lucky in California that we can camp year-round without having to only camp in the summer like a lot of areas of the U.S.
without an idea of when you want to camp and where, it's hard to give a definite answer.