When and how should I tell my new job about my vacation?

I recently got hired to work at a hotel, very recently. I didn’t really have an interview I just kind of got the job, because of that I didn’t think to mention that I had a two week trip planned for Christmas. This is a family trip that my grandparents spent over 50000 dollars on, no joke. I can not back out I’d sooner quit the job. I really don’t want to quit but I’m worried they will say no since Christmas is such a busy time in the tourist industry and I will have only been there 3 months. Also keep in mind that I am only a part time employee. I don’t know how to go about doing this, because what am I supposed to do if they say no? Quit right away and be unemployed for months? I feel like I’ve put myself into a very awkward situation. Please advise.