In your example I think that if possible those critical team members should have notified of their availability for the dates needed. The project manager should have asked if she/he did not if there were any vacation days coming up or critical project deadlines around the time of implementation. I feel like it would be rare for all critical members of a team to not be available due to unforeseen circumstances so in this example I feel like it’s poor planning on someone’s part which is now leading to possible delays in the project and potential extra costs to outsource if the project deadline is to be met.

This is a good example of why planning is so important. A good plan will also leave space for unforeseen circumstances and that’s why a late finish and early finish are included in the planning.