When the tile comes up showing what your army is as well as giving the option of fight on battle map or autoresolve and what the opposing army is, you will see a toggle at the bottom of the screen, I believe that it's on the bottom right. I am in a RTW campaign (not BI) and have not had the option of a night battle in some time.
If you can fight a night battle, you will see that the box to check is open and "checkable." If you cannot fight a night battle, the box will not be there or it will be "grayed out."
If the box is checkable and you want to fight a night battle, check the box. Checking the box back and forth you will see the picture on the tile change from daytime to nighttime images.
Strength and Honor
Celt Centurion
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