We mostly boondock, so please forgive my ignorance about reserving a spot in a campground!  About a week ago, we were kicking around the idea of going to a state park campground for this weekend, Friday through Sunday.
I got online, checking all the state park campgrounds near Denver.  We wanted a site with electricity.  I couldn't find one campsite that was available, all the reservable sites were already taken.
I got to wondering, if I was a FTer & wanted to spend a summer weekend near Denver, would I have to reserve a spot weeks/months in advance?  I read many FTers blogs who comment that they don't often reserve campsites in advance.  So what am I missing?  How do you get a spot in a state park during a summer weekend?

I believe most state parks set aside some portion of their campsites for first come first served campers. That of course would require you to come in on Thursday or very early on Friday. As you have probably read on this forum many times, most full timers avoid moving, or doing much of anything on the weekends. We are free to move and shop, and visit the attractions, during the week when the crowds are working.

You're right Fred, I have read a lot about travelling mid-week, which certainly makes sense to me.
So if you travel mid-week to a popular state park campground, I'm assuming then that you'd take a first-come-first-serve site if you intend to stay thru a weekend, because the reservable sites are already spoken for.

When we camphosted at a Wisconsin state park we posted reserved stickers on the sites reserved for the weekend and anybody that occupied them during the week were reminded that they had to be out by 11 on the reserved day. I think Wisconsin has a policy of 40% of the sites are non reservable in all state parks.

We are camp hosts in a Vermont State Park for the summer. All sites are reservable. We have been booked the last 4 week ends. There are many sites available during the week. Reserving two days in advance is generally enough notice with the exception of holidays which book a few weeks in advance.
When we are traveling we generally do not make reservations or if we do only a day or two ahead except for holidays and have not had trouble finding a place to stay. We do stay in private as well as public parks. The sites in our park are typical of the north east, they are not large.

So far Sherri and I have had no problem getting Walk up sites. We do, however, arrive at COE and State Parks on Monday...you will find that most of the time on Mondays you can get a site for the remainder of the week. In some cases we have stayed into the weekend if they have some no shows.

Thanks, everyone, for your comments!  It really helps to clarify the issue of reservable/nonreservable sites.

Coastal and near major metropolitan areas, California State Parks for the most part require reservations during the spring and summer months, even during the week. Some parks are booked solid months in advance.

We're currently camp hosting at Palisades State Park in Sterling Utah. ( thanks to H & L ) Out of 75 sites, we have 11 that are called manager's discrection, or non-reservable. A lot of the locals know about this, so they don't make reservations, they just show up and get a site. If you come early in the week and stay though the week-end, you'll usually be able to get a site. I know when we lived in Colo. the State Parks around Denver ( mainly Chatfield and Cherry Creek ) were always booked. People with money would reserve a site every week-end through the summer, then if they felt like camping they had a site. The park might only be half full, but all the sites were paid for and the state was happy. Don't know if anyone can figure a way to keep this from happening, but it's frustrating when all sites are reservable.

DZ wrote:

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....People with money would reserve a site every week-end through the summer, then if they felt like camping they had a site. The park might only be half full, but all the sites were paid for and the state was happy. Don't know if anyone can figure a way to keep this from happening, but it's frustrating when all sites are reservable.

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The federal park national reservation system publishes a No-Show policy. If you do not occupied your reserved site by check-in on the next day you forfeit it's use and  it is made available as a first come site.  If the campground reports you as a no show, you will be charged an extra $20.  

Some states have a similar reservation policy. The campground you are hosting in publishes this one: 

If you do not occupy your reserved campsite by 10 a.m. on the day after the scheduled arrival date, the site will be made available to other campers.