I don't know what to do!

I just got a email from the work agency that found me my first job...They have a 5 day job available for me the same week I have my family vacation. I have been so worn out and tired and have been looking forward to this vacation for so long. But I need money. I feel torn. I called my my mom and she said she would help me write a email back explaining that I can do it any other week, but that one. I don't want to cancel out on the temp job, but I NEED A VACATION. ...What do I do? Once the job is over I'm back to being unemployed...Once the vacation is over I will be rested and calm.  ...WTF
Am I'm being dumb by telling them sorry I can't that one week?...It's the week of July 4th.