In my last teaching job, the headmaster once sent out a list of events for which we were allowed to take a day off school. It read something like: own funeral, spouse’s funeral, own-child’s funeral, parent’s funeral, second coming.
As I stood in the corridor reading it I immediately made another list of things I would rather resign for than miss. Ever since, I have thought it was worth knowing what you would resign for.
Apparently, a certain rector in London lost his list.
Really — either say yes, and stick by it; or say no and play safe. Saying ‘I didn’t realise’ doesn’t really wash. See here.
wonderful exchange 
You’re right – doing it and apologising sends the worst possible message.