Based on reports the best days to visit Disneyland are Tues, Wed and Thurs. Saturdays are always the most crowded. There are several sites that offer traffic reports on what days and months have the most tourists, the week after Thanksgiving being one of the least crowded. During March and April various states have their Spring breaks staggered so this helps keep the crowds down, but it's still busy. The up side is during peak months all the rides are usually open and they have fireworks. During the off peak season some rides are closed for repair and there are fewer shows.
Right now Disneyland has a promotion that allows you to enter free on your birthday. Birthday guest can be seen all over the park wearing their birthday buttons. So your birthday is definitely a good day to go.
If you get a California CityPass you don't have to use all the Disneyland days in a row. For example you can go to Disneyland on Thursday and Friday, then Seaworld (which is usually less crowded) on Saturday, the San Diego Zoo on Sunday, Universal Studios on Monday and then back to Disneyland on Tuesday. These are probably the best days to visit each one of those parks, and all of them come with the Southern California CityPass. You can read more about the CityPass here: Southern California CityPass