Problems at work

So I have this job at a supermarket (I'm 18 and a student) and my co-workers just never want to help me out when I can't work. I worked three times last week even though I was sick because no one would take over my shift, I have asked a few times if someone wanted to work for me because I wanted to go somewhere really badly when I had to work, and they never would. Now there's something I want to go to so so so so badly (I would pay someone to take over my shift, seriously) but no one wants to do it. What can I do? I have helped them out so many times and they never do something in return. I would kill someone to go to this event.