The park gates close around sunset, so be certain to arrive in plenty of time.
The temperature on the mountain can drop considerably during the evening, so dress warmly. Keep in mind that this is a state park and, as such, park rules apply, including any gate fees.
At the end of the session, visitors will be escorted off the mountain. Departure will be by way of the North Gate (this becomes Oak Grove Road in Walnut Creek once you are outside the confines of the park.)
Events are only canceled due to rain or fire hazard. Don't be put off by slightly overcast skies. Often, when it is overcast in the late afternoon, by the time night arrives on the mountain viewing is fine.
If you would like advice on current conditions, you can usually reach a person in the South Gate kiosk until sunset at (925) 837-0904. For more information, call the Mt. Diablo "Hotline": (925) 837-2525. Then hit 4#.