What am I suppose to do, please help me?

I have a part time job. I do not “need” in terms of money or for experience as I’ve already been working for a yr and for min. wage I do not plan to stay there forever. So I signed up for a fun event with a friend,but it turns out it’s the same day as a big event being hosted by my workplace.

They insist I come, and are really pushing me and saying that I “have to come” to help with the event.

I’ve always done the right thing, but I’ve spent months waiting for the fun event… Just thinking about it makes me want to cry… what should I say to them, please help me