Start with the next available day
You planned to train on Tuesday, but Tuesday disappeared into a late meeting. Before moving the workout, look at the rest of the week. Wednesday might be free on paper while still being the evening before another demanding session. Moving one appointment can crowd two others together.
Check whether the programme follows named weekdays or a sequence of sessions. A sequence labelled A and B is often easier to interpret after a missed visit: the next visit continues the sequence. A coach-written plan may have different instructions, so use those rather than assuming every missed day works the same way.
Do not quietly combine the contents of two full sessions. A doubled list of exercises is a different workout, with different demands and a much longer finish time. If a rearrangement makes the week unreasonable, leaving the missed session behind is a legitimate scheduling decision.
Record the reason once
Write something specific: late train, childcare changed, gym closed. This is more useful than writing that you lacked discipline. One disruption may require no action. Several missed Tuesdays suggest that Tuesday is an unreliable training day.
Keep the completed session in its actual date slot. Moving an entry backwards to make the calendar look tidy makes later comparisons confusing. When you review the month, you want to know what happened, not what the plan originally promised.
If you train with someone else, tell them the revised arrangement before they arrive. A brief message naming the next session and time is enough; a missed appointment need not become a long explanation.
There is a useful difference between cancelling early and repeatedly waiting until the appointment has already passed. If you know at lunchtime that the evening cannot work, make the decision then. You can tell a partner, release a booked slot under the venue's policy and stop carrying an impossible appointment through the rest of the day.
Avoid making the replacement harder
There is no scheduling benefit in adding extra sets as a penalty. Return to the appropriate planned session, including its usual preparation and rest. If several visits have been missed, the question becomes how to resume at a suitable level, not how to compress the missing work into a weekend.
Finally, leave the next appointment somewhere visible. A calendar entry with travel time attached does more for the following week than a promise to make up everything eventually.