I am "redoing" and adding on to a bit of my system this year alsp. I am going to wait till March/April to start, and will start the water for each zone I install, but I will drain the water out of each line once I test it for leaks BEFORE I fill in the trench since freezing and frost still happens. This includes your mainline. A little tedious, but will pay off if you want to start early and don't want to ruin anything else.
I'm no expert..........YET!
I just like to suggest things and learn...
See what the pro's have to say first.