California Code, Revenue and Taxation Code - RTC § 4103

(1) Beginning July 1st of the year of the declaration of tax default, on the declared amount of defaulted taxes at the rate of 1 1/2 percent a month to the time of redemption. If the last day of any month falls on a Saturday, Sunday, or legal holiday, the additional penalty of 1 1/2 percent shall attach after the close of business on the next business day.

(2) Beginning July 1st of each subsequent year, on the unpaid taxes for which the property would have been declared in default if there had not been a previous declaration, 1 1/2 percent a month to the time of redemption. If the last day of any month falls on Saturday, Sunday, or a legal holiday, the additional penalty of 1 1/2 percent shall attach after 5 p.m. on the next business day. If the board of supervisors, by adoption of an ordinance or resolution, closes the county's offices for business prior to the time of delinquency on the “next business day” or for that whole day, that day shall be considered a legal holiday for purposes of this section.