Check out my review for Arch Rock from a year ago "Fantastic dry camping".

We found most any day. Lots of people come in just for an overnight and leave by 10 or 11 (sometimes earlier).

The best chance to land a spot at any campground is Sunday through Thursday. If it's a holiday, stay home.

None currently, it’s full every night.

That was our first visit, I couldn’t be sure. People say it’s very busy a lot of the time, and folks start driving around early in the morning looking for people getting ready to leave. Weekdays are more likely to have openings. There’s a BLM boondocking site just west of the park you could use as a backup if it’s full, and campgrounds in Lake Mead NRA aren’t too far.

It is busier at the weekends. Tuesdays is a good bet. We arrived mid-week and had to just wait for someone to pack up and leave, which didn't take long. So if you can, a good time to arrive is around 10am when people are on their way out.

Monday thru Thursday....definitely not the weekend.

Personal perspective: Sunday between 8 and 9 a.m.

We arrived on Tuesday, early. The campground seemed to fill up later in the day and weekends.

We showed up at 1:15 on a Wednesday afternoon and had a handful of sites to choose from. However, this was in February of 2018, so things may be a little different now that so many more people are camping and RVing. While we were there, most people seemed to leave around 8-8:30, so 9 am seemed to be the best time to arrive. As always, mid-week is probably best.

Tuesday’s then Monday’s. It’s tough because it’s such a sought after campground and a lot of people are retired and stay the full two weeks at a time. Good luck and you’ll love the area.