You might wonder how closely we stick to my 2-week menus: they're designed to be repeated the second half of the month, after all, and that means that I've scripted 4 straight weeks of meals. Don't things come up that throw the menu out of whack?
Sure they do! But, many of my main ingredients (meat, especially) are frozen, so they can be held without being wasted. Sometimes we scrap a meal altogether. Sometimes, I switch meals around or bring in something new.
For the most part, though, we do stick to the basic plan. I haven't scripted out the precise side dishes very much, so those are flexible. I know there are the bones of 4 weeks' worth of meals either planned or already in the house in frozen/pantry staple form. So, I can switch around if necessary. It's amazing, though, how closely we do follow it. In September, I switched in my grandmother's Cream Cheese Souffle as the entree for the pork roast since I had those ingredients on hand and my pork was bad. I also canceled one of the fajita meals since we didn't need it and the peppers/chicken/tortillas were in the freezer--they could wait for a bit or be used in different meals this month (the chicken and peppers from the fajitas will reappear in Chicken Scampi; the black beans and peppers planned for Chicken Tucson will reappear in Black Bean Chili).
It's worth designing menus that feature ingredients you like; if you have to change things around, you're likely to find another combination of the same ingredients that will suit you and your family.