
Summers are when you're free of classes, so can get yourself into the lab and do productive research. Most mentors are reasonable about letting you take a week or so off during the summer to visit your family or take a short vacation (not that you will be able to afford vacations

), as long as you can time it so it doesn't interfere with your experiment schedule, but if you're serious about grad school, you're definitely not getting the entire summer off...that's when you're expected to be most productive.
Think of it this way...if you weren't going to grad school, you'd be looking for a full time job where you'd maybe get 2 weeks off a year for vacation. Grad school is somewhat like taking on 2 to 3 jobs, depending on the source of your funding. You start off with job #1 being a student, and you need to take classes and study hard. Job #2 is your research, which also requires long hours, dedication, attention to detail, preparation, and a steep learning curve. Job #3, if required of you, is teaching, whether you're a TA with responsibility for grading or teaching undergrad classes, or in the lab teaching undergrad helpers. If you plan to have a career in academia after grad school (I don't know what your plans are), you should definitely obtain teaching experience.
It'll be the hardest, most sleep-deprived, challenging ~5 years of your life that prepare you for a satisfying, challenging, interesting career, which hopefully makes it all worthwhile.