...what kind of weather are you looking for?
Assuming it's sunshine, California is mild all year around. SF is the coldest out of the locations you mentioned, no matter the time of year (though LV in the winter at night is usually colder because it's in the desert). It also depends what you want to do. For example, Yosemite it best between April and June because that's when the waterfalls are at their peak. However, June is summer vacation here, meaning more crowds. It's also hotter but that may or may not be a concern to you.
All in all, I'd say that September is a great time of the year to come to California. It's after the kids go back to school but the weather is still nice and hot. But that's just my opinion... CA is warm (certainly compared to the UK) all year round. This past November we still had temperatures in the 90's down here near LA!
Off topic: I've just noticed that clicking on an emoticon always inserts it at the end of the text, not where the cursor is positioned in the textarea. That's highly irritating but it might just be Firefox (I'm too tired to check the code right now). Perhaps the main webmaster/administrator can look into it?