If you do have something planned, don't bring it up until you have an offer in hand. I'm getting married next Memorial day, and unfortunately that's going to be a month into some firm's programs. I probably won't be mentioning that I can't start until June until I have an offer, as once a Firm decides they want you, they'll be more likely to negotiate the start date (from what I've been told).
One thing that may help is to check if the firm accepts/allows split summers. They likely have a more flexible summer program.