The check in day appears to be on the week following the date you start your pills not the day you start your diet.
so for instance, if you start your diet on monday but dont start taking the pills till friday and select sunday as your check in date, you will not be alowed to check in until two sundays after, so not the upcomming sunday but the next sunday, anywhere between 7 and 10 days after you start your pills depending on the day of the week you select as your check in date.
hope this helps