Does the 15 days, per CVC 40518 (CA law), mean "15 business days" or do holidays/weekend days count towards the 15 days?

Courts will generally not engage in interpreting a statute when its meaning is clear.

"Days" means actual days unless the statute expresses something to the contrary.

Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like Avvo, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.