In CA, if a person gives a $650 deposit to hold an apartment in the beginning December with the intention of moving in on the first of Januaryand then decides that he doesn't want to move in after all and notifies the landlord on Dec 20th, is he entitled to any of the deposit? The landlord was not able to rent the apartment until the 25th January.

Was the deopsit suppost to have been used to pay your rent? Or was it just a bonus for the landlord not to rent the place to anyone else?

If the latter, then he gets to keep the money.

If the former, then he should deduct 1 months rent and give you the remainder back.

The depoisit was a good faith payment that you were going to rent the apartment on the following month.

This is to hold and save the apartmnet so they would not rent it to something else. You really should not expect to have it both ways,
If you back out, why do you even think you should get your money back.

You are in general giving notice to move out( not move in) with less than a month notice

I will agree, you will owe one months rent, if this rent is less than your depoisit, you should get back the difference, but my guess is that the deposit is equal the rent.