Sadly, plumbers and tradesmen in general, don't plan ahead.
What they do is take on six jobs at once and run between them trying to keep increasignly irate customers happy - and failing!
When we had a plumber to install central heating in our old house, it was his first job since setting up on his own. He was worried about getting work. i told him to only take on what he could actually do in agreed timescales, and let the rest go. If he couldn;t get to a job, ring and say so, and confirm when he would be back, and don't let the customer down a second time. Follow those simple rules, and he would always have to turn away work.
He came a year later to service ths system, and thanked me for my advice - said he had happy customers, a referral system of regular customers, and worked constantly - but always refused work he couldn;t get to.
It's not rocket science is it? If you say you can't come until Thursday, fine, I'll make a decision, and probably wait until Thursday anyway. That does not mean - tell me you'll be there on Monday and have four irate phone calls from me before you finally turn up on Thursday like you knew you would - but neglected to tell me.
Tradesmen - plan your jobs! You can't be everywhere, you have one tool kit and two hands - take on what you can do, leave the rest to someone else.
Customers are not stupid - tell the truth, be on time - be the most popular man in your trade.