Hi Tony -
I sent your question to the person who arranges our Paris tours, and this is her reply:
"We put a lot of thought and effort into the Christmas tours we sell – we plan them out at least initially ~2 years in advance. We consider all the options available to us and it can be tricky. Normally our Paris city tours start on either a Sunday or Monday, but often for the tours that fall over Christmas and New Years / New Years Eve we have to start on different days in order to guarantee that we can provide a well-rounded tour that runs smoothly. In France, the 24th and the 25th of December are considered holidays but just like here in the States, where the 25th is the national holiday, everything is closed (except the Eiffel Tower and a few other tourist friendly sights like the boat cruises on the Seine etc.). That, however, doesn't stop us from being able to sight-see and appreciate our time in Paris. We creatively rearrange the sightseeing we do as a group so that nothing is missed.
The Paris city tour that started on December 23rd 2014 was arranged this way: The tour started on a Tuesday, the 23rd, and on the next day, the first full day of the tour, Wednesday the 24th, we did everything as was originally planned: visiting the Historic Heart of Paris, Ile de la cité, seeing the Sainte Chapelle and Notre Dame etc.
On Christmas day when everything was closed we had to do a little rearranging from the "normal" tour schedule and had a walking tour through the Marais district. The Marais is considered Paris' Jewish quarter. Even though the rest of the city is closed for the holiday, restaurants and even some shops were open in the Marais. This was also the perfect afternoon for taking the Seine River cruise and in their free time (which is always built into our tours), many tour members took in church services, arranged to have tickets to go up the Eiffel Tower, took in the Christmas windows, or walked through the many parks (along with many Parisians who were also looking for ways to get outside for part of their holiday).
That evening the group celebrated the season with dinner together. The rest of the week went as planned for the most part. On the 26th we took the group on a walking tour of Montmartre and later in the evening we went to the Louvre, as it is open late on Fridays. Then on the 27th we visited Versailles and on the 28th Orangerie and Orsay before having one last meal together."
We hope that answers your question! Please let us know if there is anything else we can clear up for you.