Legal Question in Landlord & Tenant Law in California

If they have actually vacated the premises, they have surrendered the premises and you can't charge them rent, but you can charge them for re-keying the locks.

Keeping the keys is an indication that they have not given up possession, but so many people do not return hotel keys, rental unit keys, etc., that it is not conclusive. If all of their possessions have been moved out and they have done whatever clean up of the unit they apparently intend, then there would appear to be no reason for them to return and possession would be considered ended. They can be charged rent for the time between when they gave up possession and you reasonably would know they may have and you could re-key the unit. Also, they are liable for at least the first 30 days after they have given written notice of leaving, no matter what day of the month it is [does not have to be at the end of the month or same day of the month when they originally moved in]. By the way, if you have some turn over in the rental unit, you can buy locks that are easy to re-key without having to remove any part of the lock set.