Mont-Saint-Michel is a little island near the coast of Normandie (France). The little island is on the World Heritage List. You go to the island over a bridge, you can use the bus or walk the 2 km to the island.
What time do you go there
The day we went there the weather was not great so we had to hide from the rain a few times. It was not awfully busy anymore because most people go there really early in the morning. But after two hours you have seen most things on the island. So we went around 3 o’clock when most people had left 
already.
What to do there
We went into the church that was on the island. Which costs €10,- per person. They gave us a flyer with some information on it. It was nice not really sure if it was worth the money. But one the island it is the only thing to do. Apart from the few stores and restaurants of course.
Now one why you really need to visit once
I personally think it is really important to know how people used to live. Not only that this is a special place people used to live here without all the tourists. You will not learn how they live but the building still seems the same. So it will give you an idea.