Rick Steves's Normandy book (excerpted from Rick Steves' France 2014) states that the abbey is open until 23:00 in July and August. The official website, http://www.ot-montsaintmichel.com/en/visite-mont-saint-michel.htm, states that the abbey is open only until 19:00 in August:
The abbey is open every day except the 1st of january, the 1st of may and the 25th of december.
- from 2 may to 31 august : 9 am to 7 pm, last admission at 6 pm.
IS RICK STEVES WRONG???
I have planned my trip using the Rick Steves info. I hope he's not wrong.