California Code, Streets and Highways Code - SHC § 6570

Within 60 days after the sale of the property for delinquency, the purchaser may send to the person to whom the property is assessed for purposes of taxation as shown upon the last equalized assessment roll of the county in which the property lies, and to the person in whose name, on the date the sale is made, the legal title to the property appears by deed duly recorded in the office of the county recorder of the county in which the property lies, by registered mail and first-class mail, postage prepaid, a copy of the certificate of sale.  Together with the copy of the certificate of sale, a notice shall be sent that is printed in at least 14-point boldface type in the following form:

Your property located at __________ has been sold by the treasurer's office of the city (or county) for nonpayment of __________ improvement assessment.

You have one year from the date this notice was mailed to contest the validity of the sale.

If a copy of the certificate of sale is sent as provided in this section, no action, suit, or proceeding to set aside, cancel, or in any manner attack or question the validity of any sale for delinquency, or any proceedings prior thereto, shall be commenced or maintained by any person unless the same shall be commenced within one year after the datethe above notice was mailed.  After the year has expired, all persons shall be barred from commencing or prosecuting any such action, suit, or proceeding, and any and all persons shall be barred from asserting or maintaining in any action, suit, or proceeding that the sale, or any proceedings prior thereto, was invalid.