well one way to do this is to do it using 2 shaders.

Create 2 lambers/blinns, whatever you choose. Place your "day" texture on one, and your "night" texture on the other.

with the hypershader still open, select the faces which will be lit and right click your day shader and choose "apply to selected"

Then hold down the shift key and marquee drag over your geometry, it will deselect the current faces and select the other faces.

Apply the night texture to the faces which are now selected.

This will be a "hard" transition from texture to texture, but it's the quickest way to do it.