France's Alsace in early December

Kathy, Arriving at 4pm it will be about 5pm (17:00) by the time you have got to your hotel and checked in, and already dark. It is 17:00 now (~130 Km due south of Strasbourg), and mostly dark here already.
But, that is good, because a Christmas Market is at its best after dark. Info here: http://noel.strasbourg.eu/en/strasbourg-capitale-de-noel
Opening hours: 26 November to 23 December, on Sundays to Thursdays, from 11.00 to 20.00, and on Fridays and Saturdays, from 11.00 to 21.00

I would spend the whole of your second day in Strasbourg, lots to see in a short day.

Thank you for your reply, Chris. I just don't want to regret NOT seeing Colmar, even if it's only for a few hours. I was torn between staying the 2 nights there or Strasbourg but ended up going with Strasbourg. I've printed the Christmas Market pdf file for the markets in Strasbourg and have a pretty good idea of the layout. Of course we'll want to see the Cathedral and possibly climb those steps to the top (I've given up the idea of climbing to the top of the tower in Cologne.....ugh, 500+ steps)! I'm pretty good for booking us up for non-stop days and that's ok because that is sort of the way we travel. I have a hard time sitting still. This will be almost the end of our trip so fatigue may set in requiring us to slow down the pace.

of what you'll see on a one-day visit during winter, Colmar doesn't differ enough from Strasbourg to warrant the heading to the train station, waiting for the train, taking the train, heading into town in Colmar, and then all in reverse. Especially since it's the exact day when the altarpiece can't be visited.

If the Unterlinden museum is really important to you, is there any chance you can reverse the order of your trip?
Getting from Colmar to Strasbourg doesn't take very long at all. If the museum is what you want to see, perhaps you should make the trip.
The Christmas markets are very nice, but the stalls start to look alike after awhile - that's why I recommend seeing the Unterlinden.

I've been to Colmar but not Strasbourg so can't give an apples-to-apples comparison. However while Colmar was nice, I didn't think it had much any other village with an old town (Dinan, Rothenberg, Vannes, etc) didn't have once you take away the Unterlinden.

But why decide now? I've found over the years that some places I expect to like don't live up to what I hoped for, and others are even more delightful than I had thought. So I look up the sights where I will be visiting, but I try to come up with alternatives as well so that I have plenty of possibilities in a day to choose from depending on energy level, interest, the weather, etc.

I think there are relatively frequent trains, so I suggest looking up the times in advance and carrying them with you. Assuming you are arriving in Strasbourg by train, pause before leaving the station to look up the ticket price to Colmar on one of the automated machines so you can be sure to have the fare in cash with you. If you do this and start your day exploring Strasbourg but by early afternoon find your interest waning, then take the next train down to Colmar.

of what you'll see on a one-day visit during winter, Colmar doesn't differ enough from Strasbourg to warrant the heading to the train station, waiting for the train, taking the train, heading into town in Colmar, and then all in reverse.

It does seem like a lot of walking, waiting, riding, walking, etc.....

If the Unterlinden museum is really important to you, is there any chance you can reverse the order of your trip?
Getting from Colmar to Strasbourg doesn't take very long at all. If the museum is what you want to see, perhaps you should make the trip. The Christmas markets are very nice, but the stalls start to look alike after awhile - that's why I recommend seeing the Unterlinden.

We could possibly go on Monday but it wouldn't be until a little after 4pm. Still thinking about this one.

I've been to Colmar but not Strasbourg so can't give an apples-to-apples comparison. However while Colmar was nice, I didn't think it had much any other village with an old town (Dinan, Rothenberg, Vannes, etc) didn't have once you take away the Unterlinden. But why decide now? If you do this and start your day exploring Strasbourg but by early afternoon find your interest waning, then take the next train down to Colmar.

This is a great idea. I will print out the train schedule and keep it with me in case we decide to do this. I suppose we'll just have to plan another trip to the area. Isn't that always the way?

I loved the Christmas markets in Colmar, much more than Strasbourg, and many of the shops had not just window decorations but the entire facades were stunning. I'd head for the Cathedral on arrival and spend your full day in Colmar.

Check the closing hours of the Unterlinden before you go. Most museums start shooing people out the door 30 minutes before closing time and won't sell tickets after a certain time.

Just a thought - is there any way you could delay your train to Cologne?

The direct train from Strasbourg to Colmar takes only 30 minutes. Takes about 20 minutes to walk from the Christmas market to the Strasbourg station. From the Colmar station, it takes about 20 minutes to walk to the Unterlinden.

If the Unterlinden was open, I would say 'go to Colmar', but since it isn't, I would stick to Strasbourg

There's more than enough to see in your short time in Strasbourg. Save Colmar for another time.

I know I can always count on great advise here. Thank you everyone. We will save Colmar for another trip. I think it's best not to squeeze too much into too little time. With the 5 or so Christmas markets all within walking distance of our hotel is think we will appreciate a little down time on our first evening.

I love Strasbourg and I love Colmar. But if your trip is short, don't waste time travellng back and forth to Colmar. Enjoy the amazing Christmas markets in Strasburg. You will have an amazing time. Colmar is not that much better, in my opinion