Please tell us why our concrete contractor insisted upon placing our big wide driveway and patio at or near sunset. We live in the desert southwest and we like it, but it seemed odd to require the placement of concrete at the end of the day.
It sounds like your contractor is operating from experience gained in your desert area. I suspect the contractor is wetting the sub grade or soil with water, just prior to placing the concrete in the evening, too. This is all an attempt, in my opinion, to place and finish fresh concrete with a quality approach.
I think your contractor is waiting for the cool of twilight to place the concrete and avoid a rapid surface drying and surface cracking, normally caused by heat and wind. Heat and wind will dewater the surface of fresh concrete and leave many surface shrinkage cracks. This is a situation to avoid. I will speculate, your contractor is also spraying a moisture loss control product upon the surface of the concrete as it is being finished or "smoothed." This moisture loss product is called an evaporation control product. Our brand name is E-CON.
The next step in giving you a quality concrete project, will be the curing of the finished concrete surface. I prefer to cure with a heavy solids membrane, like our Lumiseal Plus. It cures and seals with the same product.
In closing, you have chosen a good concrete contractor, from the sounds of it.