I'm going to refuse to waste my day off preparing for a work event.
Why don't you want to prepare for the event on your day off?
I enjoy my days of rest and I don't like the idea of worrying about work while I'm trying to relax.
It's okay to not want to work during your break. There's nothing wrong with using your days off to forget about work and clear your head.
It's so annoying that they expect me to work on my day off.
You deserve to take a break on your day off, so don't worry about them. You should enjoy your break and worry about work the next day.