Most are not, but some larger Christmas markets remain open over Christmas, particularly in Belgium, but opening hours can change on 24th, 25th, and 26th December. Check the individual listing and official event information before travelling.
In Amsterdam RAI and the Ice Village are both open on Christmas Day.
Valkenburg’s two cave markets and Bruges WinterGlow all close on 25th December. Magisch Maastricht is one of the few markets in the South that stays open, from 11am to 6pm. In Belgium, Antwerp is open on 25th December but closed on 1st January, the reverse of the usual pattern.