this is from the following web site
http://www.caltenantlaw.com/Res-Law.htm
Late Fees and Grace Periods
This is an area of landlord-tenant law which most judges now understand to be different for residential tenants. A late fee is typically a flat amount or percentage of the rent that the landlord intends to charge if you don't pay the rent by a given date. Late fees are built into many contracts to encourage timely payment, but the Legislature recently amended the law to prohibit late fees in residential rental agreements. The law does not punish the landlord for trying to steal it from you.
Late fees are legally described as "liquidated damages provisions", meaning that an arbitrary amount is chosen as the penalty for breaching a particular promise in the contract - in this case, to pay the rent by the given date. Civil Code Section 1671 provides that a residential rental agreement cannot contain a liquidated damages provision unless it is impossible, or extremely difficult to calculate the exact damages suffered by the breach. If it does, the late fee clause is void and of no legal effect, just like a nonrefundable security deposit clause. Interest, in the range of 30 cents per day, is permissible, however, if the contract so provides.
Landlords will try to justify the late fee by describing the time it takes to make extra calculations, trips to the bank, phone calls, and other varying time spent due to a single late payment. Of course this is contrived, and the reality is that the management company gets the same amount of money irrespective of any late payment, and any additional work is negligible. The stronger argument is that Civil Code Section 3302 says that the damages for failure to pay an amount is the amount itself, plus interest at the legal rate [1/3650th of the amount per day - about 20 cents]. Therefore, the amount incurred from late payment is not difficult to ascertain; the law defines it.
That's the legal argument. You might want to print it out, in case you have to convince the landlord or judge of your position, and you can use it for reference.
Tactically, you face decisions on when and how to confront the issue. You can sign a lease with the late fee provision, and you haven't waived your rights by doing so, since the late fee provision is illegal and void. When faced with the demand, you can pay it under protest, or describe the money as an advance payment of rent, and avoid the immediate hassle. At a later date, if you are evicted for nonpayment of rent, your prior late fee overpayment has to be given credit, and the 3-day notice is thereby made invalid for demanding more rent than was due [ie, it demands a month, but you only owe a month minus that earlier late fee payment]. You win the case on a technicality, or at least get a great bargaining position for a settlement on terms favorable to you. If you're strapped for cash now and can't pay even the current rent, hope that you get the eviction notice that asks for the rent plus the late fee. The scenario is the same, except that you don't have to apply past overpayments for credit; the notice is invalid on its face.
There is no official "grace period" for rent payments in the law. Rental agreements typically say that if the rent is not paid by the 5th, a late fee is due. Again, since the late fee is itself illegal, threatening to do an illegal act on a given date does not help the landlord's case. Functionally, the law provides a type of grace period. If rent is due on the first, the landlord cannot give you a 3-day notice to pay rent or quit until the 2nd. The 2nd is day "zero" of the 3-day notice, so that the last day to pay and satisfy that 3-day notice comes out to be the 5th of the month. If that third day also falls on a legal holiday or weekend, your last day is extended until the next banking day, which can be as late as the 9th.
If your paycheck comes at a time which makes payment on the first difficult, you might suggest to the landlord that you increase your security deposit by a month's rent, so that you'll actually being paying a month in advance of the current one. That is, you would pay April some time in March, but think of it as March rent, even if you paid it March 15th. It would still be early. Interest lost on that money is nothing compared to the hassle you save and the tactical advantage you create for yourself by doing that, at a time when you may need it