Weather and daylight
January runs cold and grey. Daytime highs sit around 3 to 5 degrees, nights drop below freezing, and the Alzette valley in Grund can feel several degrees colder than Ville Haute. Snow does fall but rarely sticks in the city; the Ardennes around Clervaux and Vianden get more of it. Daylight is short — sunrise near 8:30, sunset around 16:45 — so plan castle visits and Bock Casemates for late morning. Bring a windproof layer for the plateau and waterproof shoes for the cobbles, which turn slick after the frequent drizzle.
What is open, what is shut
The Christmas markets close on 2 January and the Winterlights installations come down within the first week. Vianden Castle stays open but on winter hours (10:00-16:00), and the chairlift is shut until Easter. Mondorf-les-Bains thermal baths are a genuine draw in this month — locals fill them on weekends. Most restaurants in Grund and Clausen reopen around 6 January after the holiday break. Museums run normal hours. Hotel rates hit their yearly floor: expect 80-$100 at chain properties near the Gare, and even Kirchberg business hotels drop to weekend deals around $110.
Good to know
Are the Christmas markets still on in January?+
No. The three main markets in Place d’Armes, Place de la Constitution and Place Guillaume II close on 2 January. Winterlights projections run through the first weekend, then everything is gone.
Is it worth visiting Vianden in January?+
Yes if you accept short hours and no chairlift. The castle at 10:00 with fog rising from the Our is genuinely atmospheric, and you will share it with maybe twenty other people.