When someone leaves your team, there is a window from the point when they announce when they are leaving to when they actually leave. That window is the amount of time you have to get all of the training this person has learned in their time with you and download it to another person.
These are the interactions you don’t want to push off and you want to kick off as soon as possible to see what you need to learn from them.
Guaranteed as soon as you have that first meeting, you will see the immediate need for second, third, fourth, and fifth meetings you never even thought of.
And you don’t want to find that out on their last day.