How soon is it to ask for certain days off?

So I got a fast food job and I've only been working for 3 days, but next month I have a mandatory 2 weekend activity with an organization and since they told me they're mostly gonna schedule me on weekends since i'm a student, I don't know how to bring up the scheduling conflicts. I can't miss these meetings, but I also don't wanna run around and ask coworkers that I barely know if they can cover my shifts. I was thinking of letting them know 1 week ahead, but i'm not too sure on how to even word it.