I work for a german IT company where we have a monthly meet up next week, which includes a presentation e.g. about a technology. The event is outside of the working hours and not paid, it is more like a casual get together. I accepted the invitation two months ago.
Today my supervisor sent me an email, in which he assigned me for the presentation, while I am on vacation to prepare for a difficult exam (which leaves me with no time to work on a presentation), including next week.
What is the best way to politely decline this task?
Additional context: I could attend the event because in itself it doesn't take too much time.