That would be Business and Professions Code (B&P) 25620:
25620. (a) Any person possessing any can, bottle, or other
receptacle containing any alcoholic beverage that has been opened, or
a seal broken, or the contents of which have been partially removed,
in any city, county, or city and county owned park or other city,
county, or city and county owned public place, or any recreation and
park district, or any regional park or open-space district shall be
guilty of an infraction if the city, county, or city and county has
enacted an ordinance that prohibits the possession of those
containers in those areas or the consumption of alcoholic beverages
in those areas.
(b) This section does not apply where the possession is within
premises located in a park or other public place for which a license
has been issued pursuant to this division.
(c) This section does not apply when an individual is in
possession of an alcoholic beverage container for the purpose of
recycling or other related activity.
You might get a courtesy notice ... you might not. Unless you receive something else in writing, appear at the location on the citation at the date and time indicated. These are almost always handled as infractions. Handling these as misdemeanors simply is NOT cost effective, so most are taken care of in Traffic Court.