trip to 聖米歇爾山

My wife & I would go to Paris from 22/12 to 27/12. The itinerary is as follows:

22/12: set off from London and arrive Paris at 13:30 by Eurostar
25/12: as it is a holiday in Paris, we planned to go to 聖米歇爾山 and stay at there for 1 night.
26/12: back to Paris
27/12: leave Paris at 17:10 and go back to London.

Questions come:

1: would it be a better idea to go to 聖米歇爾山 during Christmas or Christmas Eve or Boxing day?
2: we would go to there by ourselves, so would it be better to stay at there for a night or just to return to Paris on the same day? Some guide books tell us that the travelling time is around 7 hrs (to and fro).

Thanks