Lots of good advice in the posts above. Personally, I would only come southbound April through Aug, DEPENDING ON WEATHER. And it's not the kind of weather that looks horrible to folks like weather routers or the talking heads on the TV, either. Summer thermal winds, caused by heating of the inland areas from CA to WA, often appear benign to those lacking local knowledge.
They see the nice Pacific High settled in, and equate that with benign conditions along the coast. Not so. Nice hot days bring up sometimes heavy thermal winds, booming CW around the highs, and generally blowing westerly off the ocean. They can raise a very uncomfortable wind chop on top of the normal 4-6' westerly ocean swell in the summer. Yeah, you're going down-wind and down-swell. But in 25+ knots of wind, surfing in a 34' trawler can be a whole new experience!
In addition, if you're considering day-tripping down the coast, one needs to consider the state of the tide, coinciding with the expected time of arrival, at each desired overnight port. If all is well, it's cake to scoot across for the night. If it puffs up during the day, or the fog settles in, or there is unexpected onshore swells, and the bar conditions are lousy, the USCG can and do close the bars with little warning. If so, it's certainly best to keep on truck'n all night, and rest sometime the next day, if necessary.
Having said all this, it's a fun trip southbound, if the boat and crew are properly prepared and experienced. If the fishermen are coming in, come in with them. If they are staying ashore, stay moored also. If the boat's OK, but the crew are novices, best to hire a delivery skipper, and crew for him. If the boat's suspect (especially the fuel system), then best to ship it. Costs WAY more, but can cost WAY less, if things go to hell in a handbasket.
Of course, YMMV. It could be a dead-flat trip like a millpond with brilliant sunshine the whole way. It can be just the opposite. And Pt. Conception is generally a piece of cake in the summer. Cape Mendocino's a bit more of a handful, IMHO.
Enjoy!
Pete