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?

Perhaps one has to spend time in Seattle to get this one...

Could it perhaps be that liquid elixir, that tends to be an acquired taste, taken with or without cream, and that provides that useful jolt of caffeine in the morning or late at night?  Oh yeah, and the houses are common - think Central Perk or similar.

COFFEE!