Some gather in my house,
because I am a part of a fad.

Sometimes I am dark.
and sometimes I am light.

I could be the savior at sunrise,
and sometimes the savior of night.

What am I?

COFFEE!!!   A "coffee house" is a fad.  Coffee can be dark or light, and it keeps you up at night, and helps you get up in the a.m.

I think this has to be it.