Three good days beat five planned ones
The most common reason a plan fails is not that it was wrong. It is that it was written for a week you do not have.
Pick the number you hit on a bad week
Setup asks how many days a week you can train, and the honest answer is not your best week. It is the week where work runs late twice and someone is ill. If you can train four days in a good week and two in a bad one, three is the number that survives.
This matters because the plan is built around the number. Say five and the week is designed so that five is normal; miss two and you are behind on a plan that assumes you are not. Say three and hitting four is a bonus rather than a correction.
Why fewer days can mean more training
Spread the same work across fewer days and each session carries more of it, which is not a loss. What you are actually trading is sessions for session quality: fewer days means more rest between them, and more rest means you turn up able to repeat your numbers.
There is also the part nobody measures. A plan you meet builds the habit of meeting it. A plan you miss teaches the opposite, and the lesson sticks longer than any single week of training.
Which days, not just how many
Choosing the actual weekdays is worth more than choosing the count. A session on a day you reliably have is a session that happens; the same session on the day you reliably work late is a session you will be rescheduling every week.
Your training days can be changed later from the calendar, and changing them adjusts the schedule without rewriting your plan or touching your history. If Tuesdays stopped working months ago, say so rather than missing one every week.
What to do with the extra day
When a good week gives you a day you did not plan for, the worst use of it is an extra hard session squeezed between two others. The best uses are duller: a walk, some easy work on a movement you find awkward, or nothing at all.
If you do train, log it as extra activity rather than inventing a session. It keeps the record honest, and it keeps the plan from treating a one-off as a new pattern it should build around.