Ask a tailor what went wrong last December and you will not hear that the work dried up. You will hear about the 20th: four garments outstanding, two clients calling, and a decision made in November that made all of it inevitable.
The commitment is made months before the failure
Every yes in October and November is a claim on a specific number of working days in December. Nobody adds them up, because adding them up requires knowing what you already promised, and that lives in a chat thread.
By the time the problem is visible it is not a scheduling problem any more. It is an apology problem.
What actually fixes it
Knowing, before you accept a date, what you have already committed and what the earliest honest delivery date is. Not a warning after the fact — an answer at the moment of the yes.
The businesses that come out of December well are not the ones that worked hardest. They are the ones that said no in November.