Good preparation is not the same as accurate forecasting. You cannot know what the week will ask for, so the skill lies in leaving space and a bit of margin instead of trying to anticipate every scenario and filling the bag to the lid. Slack is what lets you respond at all; a case packed perfectly for the trip you imagined has no capacity for the trip you get.

Apply that to a diary as much as to luggage. Leave one part of the week deliberately unspent, so there is somewhere for the unexpected to go when it arrives.