Tomorrow at about 6AM Pacific time, if you're not unlucky enough to have an overcast sky, you'll be able to observe a penumbral lunar eclipse. This happens when the Earth moves between the moon and sun, and the moon ends up under Earth's penumbra. According to NASA, the eclipse will be at its greatest around 6:33am, and will be visible until around 7am.
Because of the way a penumbral eclipse manifests, viewers will not see a defined shape, instead being presented with a shadow that grows dimmer as the planets move. This makes it a bit harder to observe; you'll have the best chance of seeing it when the eclipse is at its peak, assuming no clouds or fog get in the way.
Both the beginning and the end of a penumbral eclipse are not visible, with the visible eye being able to detect it once the moon is about two-thirds into the Earth's penumbra. This falls between the PST hours of 6am and 7am. This is the last lunar eclipse of 2012; it has a magnitude of 0.9155, according to Wunderground.com.
If you're in the mood for a bit of science, NASA has a list of all lunar eclipses in the Saros 145 cycle up on its website, which you can view via the link below. Each Saros cycle is a little over 18 years long, with tomorrow's eclipse being part of the Saros Series 145. Says NASA: "Lunar eclipses of Saros 145...began with a penumbral eclipse near the southern edge of the penumbra on 1832 Aug 11. The series will end with a penumbral eclipse near the northern edge of the penumbra on 3094 Sep 16."