December in London - Paris - Strasbourg

I would find jetlag slowing down the first couple days in London. Maybe add one night to London or you’ll see very little.

Then fly from London to Strasbourg for Christmas Eve and Day and the next day. That gives you two days for the Christmas markets. Easyjet currently has a 90-minute flight for.$56. Flying will save you the unnecessary expense & time of doubling back to Paris . While in the Alsace-Loraine. you also can visit some castles and medieval towns on the Route du Vin.
Then Paris for the final four days.
Have a great trip!

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Wherever you spend Christmas Eve and Christmas day, you may have some difficulty finding an open restaurant. There will be some, but your choices will be small, and possibly very expensive. Search this forum for comments about this and plan accordingly to avoid disappointment.
The idea of going straight to Alsace and seeing Paris in one stay is a good one. I would consider it if it was my trip.
Bonne chance!

I'm a big fan of the Strasbourg Christmas markets, but it's hard to imagine doing them in one day and then running down to Colmar. And Christmas Eve and Day in Paris will be good for walking around, but be sure you have reservations for your meals as many places will be closed, and a lot of those that are open will be offering fixed menus.

The hardest part with all of this is the fact that none of the dates have been finalized for the markets, and therefore it's hard to plan on seeing anything open after the 24th. However, with Christmas on a Thursday this year there's a good chance markets will stay open over the weekend.

Have you considered flying from London to Strasbourg, and then taking the train back to Paris? That may be a more logical itinerary.

Thank you so much for your feedback. Now we are thinking of potentially skipping London altogether. Maybe fly to Strasbourg (via London layover) for 3-4 days and take the train to Paris to spend another 6-7 days.

If we end up doing London, then take a flight from there to Strasbourg directly and spend 2-3 days (christmas eve and day at a minimum) before heading back to Paris 4-5 days. I haven't researched the Alsace region but looks like there might a bunch of things to do & see beyond the Christmas markets.

If you do that, and want another place to visit, Nancy is really nice at Christmas. Metz also has a decent market.

Or you could go north to Heidelberg, and from there it's a 3 hour ride to Paris.

Wherever you spend Christmas Eve and Christmas day, you may have some
difficulty finding an open restaurant. There will be some, but your
choices will be small, and possibly very expensive.

I spent New Year's Day 1980 in London (a LONG time ago!) and I spent Christmas and New Years 2014/15 in France. You are exactly right, there will be little open on Christmas Day (fa-ra-ra-ra-rah!) That Christmas my husband and son and I ate a Christmas Dinner that I had bought at the market the day before and heated up in the microwave! But whatever, we were just thrilled to be there, and we are happy with simple meals, so it was all good. But it is worth alerting someone to plan ahead for this contingency.

The largest Christmas market in Northern France is Amiens which is 1-1/2 hours by train from Paris. Almost ZERO tourists, all the visitors are from other parts of France, but you likely won't even see another American there - I never did! It's an easy town to get to and easy to get around in once you get there, too.