In your calendar click on a day and you’ll get a popup asking what you want to add. At the bottom is “Add an annotation”. Click that and you’ll get another dialog. The notes at the side state:
Create custom annotations to highlight things like:
Travel
Injury
Illness
Recovery Period
Change in Nutrition
Click the “Time off” option and fill out the rest as you see fit.
I think that should do it. I did the same for a holiday (vacation) for next week and Plan Builder moved everything around for me. AT then recalculated my workouts.