After going to Disneyland so many times, I do actually know quite a bit, but I'm hoping for more useless Disney info while I'm out there. Over the course of my bloggin' I will impart some of my knowledge upon you if you're looking for experienced help in planning your trip. If you're trying to find a time to visit and you are seeking my great wisdom to help you to know when to do so, my answer would be that it depends on what is most important to you.
If you want to go when there are shorter lines, and aren't so worried about how many rides are open, September to October would be your best bet. Disney actually specifies this as a non-peak period. Less rides are open during those few months (they are being repaired or prepared for the holidays), but there aren't hoards of people thronging the park. There's some fanciful language for you. Hoards. Thronging. Hehe. There is an actual list of the rides that are open at any given time in Disneyland, and if I could remember where it could be found I would probably give it to you. I know it's online...so happy hunting. Here's a list of Disney attractions though provided by Disney itself. It is quite detailed and convenient.
If, on the other hand, you are more like me and visual details are as important as rides, I personally enjoy the winter months. The whole park is decorated fabulously, and most of the rides are open. If you plan to go during Christmas prepare yourself for crowds. But, the decorations are really amazing, and there are special events during those months. My personal favorite place is N'awlins (New Orleans...) Square.
Now then, if you want to go when the most attractions are open, midsummer is generally the best time. Lots of people are there the whole summer, but with the right timing and fast passes you can hit every ride at least once, and go to see most of the shows. Nice thing about that area is that it never gets too hot or too cold. DO bring a sweater if you go because the evenings can be quite cool, but the weather is better than any other time.