What planets are visible in the night sky during the summer in California?
Depending on the hour, during June-July-August, all the naked eye planets will be visible, and with a telescope, all the major planets. With a little effort (or binocular) between 25 June to 10 July, you will see the elusive Mercury looking 15-30min after sunset, close to the horizon in the glow of the dusk, lower right, below Venus.
But I will presume, you will look with the naked eye so, just after the sunset, you will see bright Venus at the west, and in southwest, also bright, Jupiter. Then, later, at 23:00- 24:00 Mars and Saturn will rise.
During summer 2018, you mean? People have answered that.
All the planets can be seen with a telescope in California, during one summer or another. With the naked eye you can see Mercury, Venus, Mars, Jupiter and Saturn, and maybe even Uranus, during one summer or another. With binoculars Uranus is easy, tho you might need a few days to be sure you've got the right "star". Maybe ordinary binoculars are good enough for Neptune too, if you're well away from city lights?