I've had similar experiences, where I'm told in the morning to be available to handle an account deactivation immediately at X time, and then at X time I'm given the name.
What really sucks is when you have to do it for one of your teammates. :-(
Get an email the day before to deactivates someone’s access at X time. Next day they get called into a meeting 10 mins before X and then escorted to their desk and out a couple mins after.