I think the biggest problem with Southern California traffic is that it's extremely unpredictable, and when it's bad the congestion stretches for miles. Sometimes you leave 3 hours in advance and it only takes 45 minutes, and you wonder why you left so early. So the next time, you allow for an hour and end up getting there an hour and a half late.
There is some traffic on weekends near popular hang out areas since people are obviously going out on weekends, but I never found weekends as bad as weekday rush hour, fortunately, with the big exception of major events, especially sporting events. You need to know if there are any games going on: Dodgers , Angels, LA Rams games (this is a new problem, lol), Lakers, Clippers, Padres, and maybe NHL games also as well as college football (USC, UCLA).
The other major problem is traffic accidents, though you can't predict that obviously, you just have to look at Google Maps/Waze, and/or Sigalert:
http://www.sigalert.com/Map.asp?lat=33.984259&lon=-118.223015&z=2
Also road construction. I remember one time I was driving home to Orange County from LA at 3 AM, and they had shut down I-5 and diverting all traffic off the highway. Took like 4 hours. There was also a stretch along I-5 in between OC and LA that I felt like was under construction for 5 years.