For the Norcal area, the calendar doesn't get updated with race dates until early August or so. Even then, race dates will change.
So, check the calendar and see what the promoters require for licensing. For example, CCCX series didn't require anyone to have a license but the other series, like East Bay, Pilarcitos and Velo Bella, did. Also, in the past, you could pretty much race any category you wanted but now, USA Cycling has set up categories, which is similar to the road categories. But, in your case, it might be simpler to buy the one day license then decide which race you want to do. From what I'm seeing on the local promoters aren't really for category idea so they might just let you race in whatever category you want. But, I'm sure this is going change in the coming months.