A small group will get there Monday on the dot, but you to don’t need for a spot in shade on the river.
However some will come from Denver and around Monday or Tuesday night to set up camp, and then go back to work for a few days, so for that big of group if you wish a shady spot you might want to get there by Wednesday if not before.
However the middle of the field at Meadow Park can be gotten at any time before the music starts for a group that big, you’ll just either need to bring shade or not care about lacking it!