California Code, Insurance Code - INS § 662

(a)(1) A notice of cancellation of a policy shall not be effective unless mailed or delivered by the insurer to the named insured, lienholder, or additional interest at least 20 days prior to the effective date of cancellation; provided, however, that where cancellation is for nonpayment of premium, at least 10 days' notice of cancellation after nonpayment of premium due by the specified due date accompanied by the reason for the cancellation shall be given. Unless the reason accompanies or is included in the notice of cancellation, the notice of cancellation shall state or be accompanied by a statement that upon written request of the named insured, mailed or delivered to the insurer not less than 15 days prior to the effective date of cancellation, the insurer will specify the reason for the cancellation.

(2) A cancellation based on nonpayment of premium shall only be effective on the date specified in the notice if the insured has not cured the nonpayment of premium due identified in the notice by the end of the 10-day period.