If you want to vote by mail, you must be registered to vote at least 15 days before Election Day.
Conditional Voter Registration
California law allows citizens to register and vote after the registration deadline. In order to qualify, the voter registration application must be submitted in person at the Registrar of Voters or an Early Vote site on or before Election Day, or at a polling place on Election Day. After completing a voter registration application, the voter will be provided with a provisional ballot.