This is a good question, and the answer is usually "It depends on what you're planting." But there are some pretty good general rules of (green) thumb to follow:
In general, early fall is considered a good planting time for most (but not all) plants since the soil is still fairly warm (to promote root growth), the air temperature is cooling down, the days are shorter, and the angle of the sun is lower in the sky. Plants have a chance to develop stronger roots and to get established without having to transpire much. Planting in fall also reduces the amount of supplemental irrigation a plant needs (due to the lower air temperatures, shorter days, and angle of the sun) so this is good for water conservation.
Plants that are frost sensitive (citrus, avocados, bougainvillea, fuchsias and succulents) might do better if planted in the spring after the last frost date has passed.
Most California natives and Mediterranean plants will do just fine if planted in the summer, but it means that they will need to be watered more frequently. Even if natives are planted in the fall, they will still probably need some supplemental irrigation through their first or second summer.