Let me get this straight - they had the planning meeting in September for an event the first weekend of December and he was expected to remember?  Was there no follow-up meeting after the September meeting that he would have been at least invited to?  No meeting notes or action items that needed to be discussed?  

A manager cannot be expected to "remember" something like this for months when they have a full schedule to concentrate on.

A better response from the Dir. of Ed's assistant would have been to probably state they had the meeting in September and a follow up on X (that maybe he missed) and ask what could be done different to keep him better informed (along with suggestions such as having you involved or at least copied on e-mails so you could help with follow up on his participation).

I have a boss that readily admits that he "forgets" and it is part of my job to nag at him to remember certain things.  I am able to get after him about not reading e-mails or paying attention to their content (its a pet peeve of mine and an ongoing issue I have with most of his direct reports when I send out info on his behalf).........BUT it is done in a light hearted way and he knows I take into consideration why they might not be read.  And knowing that they don't always get read is why I will go and ask him if he did read it as part of my follow up.  

Michelle