Eastern Europe is your best bet because some of those celebrate
Christmas on Three Kings Day (Jan 6). The Baltics, Prague and Budapest
would be where I would consider looking.
Here most of the markets stay open until the 1st or 7th of January. While I suspect inventory is low, I also wouldn’t characterize it as a fire sale. It could be characterized as soaking the tourist money for one extra week as the locals usually pack it in on the 24th (or at least here). The extension to the 7th if fairly new, the last few years.
I am not familiar with any Eastern European country that as a national policy or as a policy that represents most of the Christians in the country which “Celebrates Christmas” on January 6.
They may have a celebration for Three Kings Day, and it might even be large (but that is more likely in Spanish speaking cultures), but it isn’t a replacement for Christmas, especially in the very Roman Catholic Poland, Czech Republic and Hungary. They do have a festival and a parade in Prague and Warsaw on the 6th. I attended on in Rome as well a few decades ago. So to the extent that Eastern Europe will still have the markets going when maybe places in the west (I am curious about Spain though), its not because they celebrate Christmas on another day.
Some of the Balkan states and a few others do have a different tradition for what is Christmas Day. These would include Georgia, Moldova, Bulgaria, Kazakhstan, Macedonia, Montenegro, Greece and three others. But it’s not that they are celebrating Christmas on another day than us, it’s that they use a different calendar in the Orthodox Church.
Of course I have been wrong before and despite a hurried attempt to check my recollections (and I have done two 7 January Christmas' in two different countries), I could still be wrong.
Here is the poop on our markets. These are the 2023/24 dates so you will need to adjust them to the corresponding day in 2024 (usually a Friday or a Saturday) And a walking tour of the markets in Pest https://maps.app.goo.gl/P3e9w6Jcg7Vy3ATz6
VOROSMARTY TÉR
17 November – 01 January
With 100+ stalls, the largest of the markets in Budapest.
FASHION STREET
DEÁK FERENC UTCA
17 November – 01 January
ERZSÉBET TÉR
17 November – 01 January
ST. STEPHEN’S BASILICA
17. November – 01. January
The second largest in Budapest and the one that has been awarded as the best in Europe on four separate occasions. https://dailynewshungary.com/congratulations-this-hungarian-christmas-fair-is-europes-most-beautiful/
VÁROSHÁZA / CITY HALL PARK
24 November – 7 January
MADÁCH TÉR
3 December – 22 December