In this part of northern Colorado, in USFS CGs, you'd have the pick of the litter for Sunday through Thursday. Before I retired, my days off were frequently Sunday/Monday/Tuesday, and I would have just about any CG I went to pretty much to myself, and could take whatever spot I wanted. I would leave around 11am on Sundays, when everyone else is coming back, and set up before 2pm.
For CO State Parks in this area, it's somewhat similar, but there can be a bit more competition for spots. Again, if one leaves on a Sunday when everyone else is going home, it makes it easier to find a first come/first served spot where someone may have a reservation for the weekend...just not for the first of the week.