You can do a google search and find a countdown calendar type of thing that will tell you, or set up a countdown widget on your phone. If you have a My Disney Experience account online (which you will need if you want to make your dining reservations online and your FPs online), after you log in, on your welcome page it will tell you how many days are left until your vacation. Also, if you are booked through Disney, you will get a booklet in the mail with your dates for dining reservations and FPs listed in the back.
Hope that helps!