By default, the weekly meal planner will automatically run once a week, and send you your meal plans by email the day before your chosen grocery shopping day. You can't change the interval that the automatic planner runs on, but you can still generate plans for 1-2 weeks and then modify your grocery list to be for any days that you want.
So first, generate your meal plans by heading to the week in your meal plan you wish to generate and tapping the "Regenerate week" button at the upper right of the site:
Then when you want to get the grocery list, go to the Groceries page by clicking "Groceries" in the left sidebar and modify your date range to what you want the groceries for by clicking on the calendar button:
*If you'd like to change your grocery shopping day, you change the day of your week your plans start on by clicking on "Generator Settings" in the left sidebar, then changing the day of the week your plans start. (Your grocery shopping day will fall the day before your plans start.)