I've always thought it was CRAZY to start promising any day of the week to customers. Inevitably you have delays, weather, etc, and you wind up running around in circles to stay on schedule. The better lawns I put towards the 2nd half of the week, the weed patches and such were mowed early. And what happens if it's 330 pm on a Wednesday, you've finished ahead of schedule, and you are 2 minutes from a home you have promised to do on a Thursday or Friday? You gonna pass it up?
It just gets complicated. I have had a few late-week requests over the years, but I find they don't make as big a deal about it if you don't bring it up first. "once a week before the weekend" is good enough. Just use common sense and get a sense of how lawn-conscious each customer is. The guy who dotes over it and has a fine lawn will probably notice it's getting longer by Saturday, so mow him Wed or later. But granny crabgrass patch isn't going to care, or shouldn't. Commercial could also go up front early in the week, since they're there on weekdays to enjoy it.