All meal plans start on a Monday, but you may need a few days to shop and prepare.
Once you have added a meal plan, click the > next to the meal plan, and then the green "start" button. By clicking the three dots to the right of the "current" button, you can edit your meal plan to your needs. For example, adjust the servings, swap recipes, or delete recipes until you are satisfied. Once you are satisfied, go to the shopping list icon on the left of the green "current" button. Add to shopping list. Check off all ingredients that you already have on hand.
You can read more about general meal plan navigation in this Help Center article.