California Code, Revenue and Taxation Code - RTC § 2705.5

If December 10 or April 10 falls on Saturday, Sunday or a legal holiday, the time of delinquency is at 5 p.m., or the close of business, whichever is later, 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.