My travelling companion chose the date--cruising along the Seine in December.
Upside, France is wonderful anytime. We had cool weather but the decorations and the lack of the tons of tourists around during warmer weather made up for it, at least for me.
Moored alongside the quai within walking distance (15 min)of the Eiffel Tower was almost magical. It was great in the evening when the lights flashed on the Tower. Downside was that the metro took a fair bit of time to get into town. It is certainly doable but the metro station for the Quai de Grenelle did not have an elevator available. Also the one wicket closed for lunch so you needed change to buy a ticket from the machine during the clerk's lunch break.
Lovely to have many excursions included in the package price as well as wine and bottled water.
Local and ship guides were all very good. Apparently some people had a problem with the French accents but not me. (in Auvers sur Oise)
Rouen is a super port with the ship moored not too far from town. Good to wander at will after the included excursions. Going ashore in the late evening after dinner showed that the town seemed almost deserted in the Dec. chill, though.
My favourite day was visiting the Normandy coast with the "Commonwealth" tour including a trip to Bayeux to see the famous tapestry.
Sailing leisurely with some cultural activities (including cooking options) thrown in is a pleasant way to travel.
Because of heavy rain (umbrellas provided for tours) the river level was too high for us to journey back to Paris and pass under the various bridges as planned and we ended our trip in Conflans on the outskirts of the city. So, don't count on a return trip to the city. Some folks had dinner, theatre reservations and didn't get there. You can go by train, apparently it isn't too far, but...not as convenient.
France in December....why not?