The best way to ask for 2 days off of new job?

ok well i started this new job on may 27 and i work part time ive only been working 3 days a week so far and been taking the extra hours they ask me to stay, but way before i got this job i paid for tickets to go on a vaction for 4 days so that weekend i wont be able to work and ive paid alot of money for this and have been looking forward to this for almost a year now, so do you think that its wrong for me to ask for 2 days off of my new job for this??? and what do you think i should do/say??