The law does not specify when a late fee is due--it's based entirely on the lease. If the lease is ambiguous or unclear as to when it's due, you might be able to fight the fee...but since fighting it could involve litigation (a lawsuit), such as if the landlord tries  to sue you or evict you over the money, it's unclear that it would be worthwhile to do so. If there is at least a chance the landlord is correct in his/her interpretation, then it is probably best to pay the fee and be sure to pay the rent earlier in the future.