It is the day of check out, not check in.
I am assuming that you want to call right at the opening of the 7-month window.
Let's suppose, for example, that you are arriving on November 8 and departing (checking out) on November 15, then you could call on April 15 to book the entire vacation or start calling on November 9 to book the reservation day-by-day, telling MS on the first day what your final check out date is so they'll know to hold off the reservation mailing until after you have added on your entire vacation.
With the latter scenario, you'd call on the 9th to book the evening of the 8th, call the 10th, to add the evening of the 9th, call the 11th, to add the evening of the 10th, etc.