Once you've made your start date selection, the date can't be changed, but you can still enjoy the meals generated for the next 7 days, regardless of the date.
At the end of the week, there won't be a check-in, but whenever you're ready for new meals for the following week, you can simply regenerate your plan.
To do this, just hold down the gray bar at the end of your meal plan for any day of the week for 15 seconds. This will refresh your meals for the entire weekly plan.