The good ones
Place d’Armes hosts the main 'Wanterlichter' market — traditional wooden chalets, mulled wine ($6), Gromperekichelcher (potato fritters, $5), and a small ice rink. Open late November to 24 December, roughly 11am-9pm. Place de la Constitution has the big Ferris wheel with a view down the Pétrusse valley ($8 a ride) and food trucks — go for the view at dusk.
The skippable ones
Place de Paris market near the station is small, feels sad, and is mostly just three stalls. Rue de la Poste's little installation is essentially retail decoration. Focus on the two central squares, add a stop at Neumünster Abbey in Grund for their concerts. Hotel prices actually drop in December (30% off summer) which is the best-kept secret of Luxembourg winter.
Good to know
Are the markets open after Christmas?+
The main markets close 24 December. A small New Year's market runs 26 December to early January at Place d’Armes but it's about a third of the size.
Do I need a jacket?+
Yes — December averages 2-6°C, often damp. It's not brutally cold like the Alps but the humidity cuts through. Layers and waterproof shoes.