There's a reason the Anaheim area used to be covered with citrus groves—the Mediterranean climate is generally warm and sunny all year, with the best weather in spring and fall. Even in midwinter months, the temperature can reach into the 80s F/high 20s-low 30s C. Temperatures can drop several degrees at night even on the hottest summer days. So if you plan on being out well after dark, take a light jacket or sweater for chilly evenings.
Occasionally, fog rolls in during early summer months (referred to by locals as "June Gloom"), when mornings are often cloudy and the temperature can be surprisingly cool; hot winds—the Santa Anas—are common in the middle of autumn and early winter.
It's worth noting that daytime temperatures in summer can soar, and that air quality often declines in proportion because of smog. Rain is relatively rare, averaging fewer than 40 days a year; it's virtually unknown May-November.