Have your parents check this web site for your graduation date...RTC posts the upcoming dates...so as soon as you get there have them start checking...that way you will know for sure! This also will give them an idea on who the day will go and how many guests you are allowed...When you get there you will know your division number so they can look to see when your graduation is...you can always call them to let them know faster, but I remember getting a letter from RTC with all the info a about 2 weeks in to my sons bootcamp.
http://www.bootcamp.navy.mil/upcoming_grads.asp