TZ zone Europe/Brussels has a transition from WET to CET on 8 November 1914:
                        0:00    -       WET     1914 Nov  8
                        1:00    -       CET     1916 May  1  0:00

What is the source for this information?

Shanks, Henri le Corre "Regime horaires pour l'Europe et l'Afrique' and Gabriel "Trait de l'heute dans le monde' have all
4 August 1914, which was the begin of the German invasion.
It seems unlikely that the whole country switched to German time at the begin of the invasion, and therefore I do not trust these three sources.

Brussels was occupied by the Germans on 20 August 1914.