Please forgive my ignorance, but do Thais have a winter holiday week at the end of December that coincides with Christmas and the western New Year? It seems as if Bangkok Air flights to Siem Reap are super-booked starting December 26th. If that is the case we might move our Angkor trip to earlier in the week.

I'm not sure how relevant your question is to your concern. You see that the flights are full or are getting there and because of that you must assume that they will sell out soon (including earlier in the week).

Which means that you ought to buy your airline tix as soon as possible.

We have our tickets and also have alternative reservations on what seem to be less busy days. I was just wondering if locals will be traveling during the Christmas week or if it will just be foreign travelers.

To clarify - My concern is that Angkor is going to be way more crowded after Christmas than before. I thought someone might know if that is because the Asian winter holidays start later than ours.

January is right in the middle of the high season for Thailand (and other places in Asia). Holidays, hotels, flights should have been booked months ago.

Maybe this helps ( for Thailand anyhow ):

A very large number of those who visit Thailand over Christmas do so for a 'couple of weeks', so there tends to be a 'surgre' of people arriving just before Christmas and heading back in the early days of January.
This year Chrsitmas is on a Monday so I would 'guess' many from say Europe, ANZ etc would be heading down here over that weekend, so for international travel that will probably be the very peak days.

Now, Thai people and other residents would not be traveling that much over that time although there will be 'some increase' for those who are taking a 'New year' break and want to head off early.
New Year is different, a large number of people will be heading off and out of Bangkok 28th/29th and busiest of all that weekend. Heading back 2nd/3rd.

So that whole period is extra busy.

Angkor is for those of us in this part of the world a 'very' expensive destination to fly to, so you may have more tourists there but many will be from other parts of the world.

What you really need to make sure you have well sorted is any flights/transport between 29th Jan and 2nd-4th Jan as that is when people will be moving around, it is also absolute peak for hotels.

If you already have confirmed seats and hotel for hotel in Angkor and confirmed flights my personal opinion would be not to risk changing, even if you can change a flight that is no good if you can't change your hotel rooms at the same time, you could get into a 'mess' if you say changed flights whilst at the same time trying to change hotel rooms.
If you have flights and rooms confirmed I would stick with it as trying at this time to balance changing hotels and flights you could get into confusion.
Make sure any other flights and hotels you have confiemed and fixed.
Hope that helps.

I mean 29th Dec and 4th jan.

....and of course if you change the Angkor trip that also means changing hotel dates in Bangkok and anywhere else you are heading to, I would not risk that at all!

Thanks, James. This helps a lot. I do have duplicate reservations to SR (both air and hotel) for two different 3-day periods during our trip. We arrive in BKK on 12/20 and leave for Japan on 12/28, so it looks like we will miss the most hectic period.