I am a San Diegite, and on April 4-11 San Diego is having its first anime con, Anime Conji. I don't know if I'll attend because I was invited to a wedding on that Saturday.
Around Memorial Day is a San Jose con, Fanime. It's probably the biggest fan-run con ever.
4th of July weekend is Anime Expo (in LA). If you go to no other con, go to that one.
There's also Comic-Con (also San Diego), which to be honest is getting less and less fun since going there feels like an asphyxiation. This is usually in the end of July.
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